Stalling / CPI question

My old 93 S-10 Blazer has a idle problem. A little history, it has a 4.3 Vortec "W", 98K miles on it. Had the fuel pump and ignition module replaced this past winter. Due for new plugs and wires although they look OK, (30K on them). Distributor cap replaced in January. It started stalling on me at slowdown /stopping on a very rare occasion, but would start right up. Getting more common. Idles at 650 in park, 600 in gear. Gets 21-22 mpg, runs good at normal speeds. Worse now with the A/C being run. It seems to idle "weak". I have hooked a scan tool to it and found nothing out of the ordinary except idle flutuates 30-50 rpms, but replaced the O2 (was past due for change) cleaned the EGR, IAC, and areas around, no improvement. I only got one trouble code the day after installing the O2 and it was for running rich. Nothing else before or since. The truck seems to idle well with a slight miss , then stumble and can sometimes catch itself and the idle jumps up for a few seconds, but most of the time it stalls. While I am still checking it out (going to do fuel pressure test later this week) I did pull off the Vortec plate and removed the round sensor and looked around for trouble. I notice that one of the injectors had a very small puddle around it and there looked like a very small washed area nearby, but hard to see a lot. My question is this. I have seen a lot of great information on here on the CPI and opinions as well. Seems to be people complaining at times that the system still shows signs of leaking after repairs are made. Maybe they installed wrong or rebuilt stuff? Under normal circumstances will these things leak a little, or does ANY fuel in there pose a problem? I suspect the injector nearby is leaking and if that is the case it will have to be dealt with when I get the money to fix it. There is no gas in the oil, I always check for that constantly since first getting it. Thoughts?

If you haven't already torn into the upper intake on that Vortec, and fuel pressure/volume checks out; it's more than likely a regulator or a fuel/poppet injector line that's gone south on you. As for getting the right parts/revised parts, GMdirect.com's got the goods. May still have to hunt down the nut kit/injector feed and return lines at the dealership. The only reason I could see for having leaks after doing the job is not double checking all of your little o-ring seals that come in the kit. *Always* replace *all* o-rings and if you're missing one when disasembling parts, check inside the bores of tubes with a light probing with the end of a hooked pick. I *think* I am seeing a leaking regulator diaphragm myself in my truck, the idle's starting to lope and the truck has a hint of "gunpowder scent" as it sits in traffic or idles at the curb in the morning. *sigh* Rather than just replacing the seperate regulator, I am going to get all of the gaskets, seals and associated parts so everything is fresh. Check out BlazerLT's "CPI Injector Repair Day" thread in this forum if you already haven't. There is much good advice, imperical and anecdotal to be found there. Once I get my finances in order from repairing the entire cooling and lower intake fiasco of the past two months, I will be ordering my nut kit parts and a complete CPI system w/ updated, late manufacture parts. I hope it's the last time I have to look at it for another bazillion years. ;)

Replace the CPI and nut kit.

The pool of fuel is from the fuel pressure regulator leaking on it. replace the nut kit, the cpi injector and use a new upper plenum gasket for it all.

No one has seen a leak after replacing it other than maybe a couple.

Also, you have a vacuum leak like I had causing the idle surge. This is the IAC valve trying to compensate for the vacuum loss.

Replace all the vacuum hoses other than the PCV valve hose and the EVAP solenoid hose which probably not need replacing.

There is a hose that comes off the back of the plenum near the distributor cap and it goes to a T which then goes to the vacuum ball for the HVAC controls.

Replace all the hose. The hose is cheap and is 5/32 if memory serves me correctly.

Also, did you replace the rotor when you replaced the cap?

Also, did you replace the rotor when you replaced the cap?

Also, did you replace the rotor when you replaced the cap?

Also, did you replace the rotor when you replaced the cap?


Excuse me, Caller? Could you please turn your radio down in the background? Thanks. ;) *Duck-n-RUNS away...*

Yes, replaced both when distributor replaced. I am going to d/c it anyway as I put about 20K miles a year on it. When the distributor was replaced the vaccuum hose you spoke of fell off and I replaced some of the hoses in it then. Was causing idle issues and that cured it for a few months. I have gone over the other lines pretty well, but, it makes sense to replace them all anyway if I have to replace the other stuff. Will have to wait a little while as the old lady lost her blasted job (this kind of crap just seems to always happen when times are tough), but I won't let it go long. I have to drive it some, but with gas now at $2.65-$2.69 locally I plan to car pool several days a week. I still plan on checking the fuel pressure later this week when the weather lets up a bit. One thing I wonder, are the new cpi units improved over the old ones ? Thought I saw an ad on Ebay where someone stated these were indeed improved units. Or, are they just good for 100K? Just curious.
Also, i will check out the GMdirect.com. Thanks for the advice.

If you have gas pooling in your plenum, your CPI needs replacing badly.

If you keep on driving it like this, you will eventually ruin your engine seeing the gask goes down into the oil, thins it out and ruins your main crank bearings.

Also, the raw fuel will destroy your cat.

Just curious, but with the fuel dilution of the oil, would one readily percieve a rise in oil level on the dipstick in addition to the raw fuel smell of the oil? I am not seeing either, but that may change quickly. I haven't gone into the intake since I last replaced gaskets and seals nearly a month ago, but didn't notice washing, it was a solid black, sooty-grimy mess. Before I go "tweaking" in there, I am going to purchase a new fuel filter (it's due to have it changed again) and a pressure gague for a leak down test and a fresh plenum gasket before I do anything else. (The gasket is cheap, so looking aint going to hurt that much) I am crossing my fingers and praying she holds together for 2 more weeks when I get another paycheck in.

Just curious, but with the fuel dilution of the oil, would one readily percieve a rise in oil level on the dipstick in addition to the raw fuel smell of the oil? I am not seeing either, but that may change quickly. I haven't gone into the intake since I last replaced gaskets and seals nearly a month ago, but didn't notice washing, it was a solid black, sooty-grimy mess. Before I go "tweaking" in there, I am going to purchase a new fuel filter (it's due to have it changed again) and a pressure gague for a leak down test and a fresh plenum gasket before I do anything else. (The gasket is cheap, so looking aint going to hurt that much) I am crossing my fingers and praying she holds together for 2 more weeks when I get another paycheck in.

Small amounts won't raise the level nor will they be detected by smelling the dipstick, but they will be doing damage internally.

10-4 - I will investigate very shortly. Thanks for verifying a thought/fear.

I suppose I will just park the old truck for a while and drive the wife's Intrepid when I need a vehicle until I get this fixed.(Heck she does not need it while out of work anyway). The amount of gas around the injector was so tiny that it was hard to see at first but as you say even a small amount of gas could screw it up and my plans call for me to drive this truck for a long, long time. Once I get the plenum off I may find more of a mess than I can see now too. I will study up in my manual on this procedure and revisit the shortcut to LT's CPI changing on here as well. I may have a couple of questions later as I get ready for this. It is due for front brakes and the rotors need replacing too so I guess I'd better try to borrow a few bucks from the credit union and get this all done.
One question though, on a slightly different note. I want to check the timing on this truck eventually. I do not have a timing light. I have looked at a lot of them on Ebay but see some that say inductive. Others do not. May sound stupid, but is there more than one type of timing light? Any recommended?

There are some inline timing lights that are merely a fancy flash-tube device that fits between the plug and the spark boot. However, given the sensitive nature of the ignition systems and timing in these trucks it's best to use an inductive pick-up light. Just about any one that says it's inductive will do... clip the red and black terminals to the battery and then clip the pick-up on cylinder 1's spark wire. Make sure the engine is fully warmed up and that there is no Service Engine Soon light on. Pull the passenger side carpeting up just to the side of the front kick-panel and you will find a tan wire with a black stripe with a little connector in the middle of it. Disconnect this wire from the in-line connector, this removes the computer's control of the engine timing so you may set the base timing of the engine. Now for the fun part - you're going to need a wrench to loosen the distributor bracket so it can be rotated if timing needs to be adjusted, but not so loose that you completely loose timing. You want it to turn only when firm pressure is applied to the distributor. Start the engine and keep the transmission in park with the brake on and wheels chocked; hopefully keeping the timing light wires out of your fan and belt - aim it at the front of the engine on the main crank pulley. It's easiest from the driver's side of the engine. At the 2 o'clock position behnd the harmonic balancer, you will find the timing marks. Adjust the distributor in very slight movements as needed until you get it dead on 0 degrees of TCD on those marks. Cut the engine, tighten the distributor bracket, remove the light and plug your timing wire back in and lay the apholstery back in place. You're done. If timing jumps randomly a few degrees here and there, suspect valvetrain/timing chain backlash that's way out of tollerance or a loose/worn timing chain. Hopefully, she just simply works and behaves well for you. *crosses fingers*

I suppose I will just park the old truck for a while and drive the wife's Intrepid when I need a vehicle until I get this fixed.(Heck she does not need it while out of work anyway). The amount of gas around the injector was so tiny that it was hard to see at first but as you say even a small amount of gas could screw it up and my plans call for me to drive this truck for a long, long time. Once I get the plenum off I may find more of a mess than I can see now too. I will study up in my manual on this procedure and revisit the shortcut to LT's CPI changing on here as well. I may have a couple of questions later as I get ready for this. It is due for front brakes and the rotors need replacing too so I guess I'd better try to borrow a few bucks from the credit union and get this all done.
One question though, on a slightly different note. I want to check the timing on this truck eventually. I do not have a timing light. I have looked at a lot of them on Ebay but see some that say inductive. Others do not. May sound stupid, but is there more than one type of timing light? Any recommended?

You won't see a lot of fuel seeing there are drain holes that drain anything in the upper plenum down into the two middle cylinders.

Fuel pressure tested out this way. When the switch turned on, shoots to 60lbs, then zero. I assumed it would hold presssure with key on, but, so much for assuming. When started it hits 60lbs for a second then drops to 54 lbs where it stays. Plugs turned out to be very worn,(thought I put platinums in, but did not) cap/rotor are OK but show signs of age. Plug wires are OK as I thought, but I am debating changing them anyway... They are < 2 years old, but have 34K on them. I plan some major duty work for the old truck in the next 2-4 weeks, including a total tune up including fuel filter, CPI assy, and brakes. Just a thought, does the pressure seem OK as I have listed it? With the pump being 8 months old I thought it would be better.

It should never drop like that.

Replace the CPI and the nut kit and use a fresh upper plenum gasket. also clean your EGR hole which will be exposed when you remove the upper plenum.

Replace the fuel filter, air filter , PCV valve along with the plugs, cap and rotor, wires etc..

Make sure the coil plug wire connection is clean and tidy and remember to transfer the wires to the new cap one at a time.

This is addition to my original post. I am trying to d/c the fuel pump to be sure it is not also a problem here. I have been working on the old truck as I get money to work with, new brakes, tune up parts, etc. I have put ut the CPI on hold until early October as I still am still saving money for it. Been making due with old ladies car. I am suspect of the fuel pump that was installed in this truck in Dec/Jan. Noisy as hell, but I see many are so I have ignored it until now.
I know there is a fuel pressure issue here and most likley the regulator, HOWEVER, would apprciate feedback on the fuel pressure tests I have done to it to be sure it is not BOTH.
When ignition switch turned on, pressure goes to 60lbs for the 2 sec the relay allows it to run without starting. then drops to 58lbs and stays there. No bleed off that I see after 5-6 minutes. When you start the truck it drops to 54lbs. Will go up when you give it the gas to around 60lbs and drops back again to 50-54 then settles at 54. How can I be sure of the fuel pump with these readings?

Nothing wrong there.

But it is the leaking of fuel into the engine is what you have to be worried about.

You run it with fuel in the oil and you will ruin your engine.

You know you are right there on the pressure. Darned if I did not miss read my service manual on this. Can't believe my mistake. According to the manual it is AOK as I listed it. For some reason though today's reading was better than before. It had been dropping from 60 to 20-25 lbs instead of 58 lbs with switch on. Perhaps my valve was dirty or did not have the tester screwed in far enough. I plan on using a special flashlight to d/c my earler observations on the CPI. I was using a light that made it difficult to see well, and the fuel I thought I saw may have been nothing. The wet area I refered to is so tiny it may just be a oil spec, not pooling.Very tiny! Can't see well enough to verify washing as is. It is just so hard to see correctly. This new light has a special "light wand" bend feature that will shine into plenum ONLY, while keeping the body of the light out of the plenum, and I should be able to see better. If it does, I will send a pic of it for anyone needing to look in theirs. Also, just for the heck of it, are you aware of any recalls GM had on these CPI's?? I ran accross a referance on a recall number 99034A in 1999 on my truck that was simply listed as CPI clean/replacement. I am going to call local GM stealer in the morning, but this intrigued me. Was there a recall on ANY GM CPI that you know of? Of course, it being 1999 maybe too late, maybe mine was replaced before. Who knows?

No, what you are seeing is a faulty regulator that is not holding pressure from time to time.

When you see the drain off of pressure, the fuel is probably dumping into the engine.

You might want to check out ebay for the injector. Save some money.

It should never drop like that.

Replace the CPI and the nut kit and use a fresh upper plenum gasket. also clean your EGR hole which will be exposed when you remove the upper plenum.

Replace the fuel filter, air filter , PCV valve along with the plugs, cap and rotor, wires etc..

Make sure the coil plug wire connection is clean and tidy and remember to transfer the wires to the new cap one at a time.

make sure the coil wire does not touch the trans dipstick. tie it up out of the way. when i first got my truck the wire rubbed on the dipstick tube and caused a slight miss. i used a wire loom to space it up off the tube.

I am more confused by this old truck than ever. I got a very good light and made a better check of the plenum than before (looking under the large valve under the Vortec label). This light let me see MUCH better and here is what I see. What I thought was gas was a small amount of oil, most likely being sucked into plenum by normal means. I took a sample of it, and it is oil. I see nothing leaking, however I may need to remove the plenum to be absolutley sure about this. The regulator did not show any visable signs of leaking, (that I could see), and the only area that looked "clean" was a small area on passenger side, but it is not what I would think of as being washed with gasoline. I let the pump build up pressure and checked every so often, no signs of gas leak. I realize this is not conclusive, but, I have a question. "Assuming" that the CPI is not at fault, would poor timing result in a condition of stalling in gear. The truck can idle with not problem as long as it is not in gear. Idles @ 650rpm, and goes to 600 (per computer) when in gear. Will idle just fine, then all of a sudden - it dies. Starts right back up, fuel pressure remains constant when this happens, not dropping first. The truck had a new distributor installed this past winter, and I was not able to do it myslef as it left me stranded in town. Is is possible that the timing could be off enough for this to occur? the fellow who changed it is supposed to be pretty good,(he maintains our GM trucks for the company I work for) however I found that he had left the cap loose on one side and he pinched the wiring when putting the cap back on. Clearly he was in a rush on this as it was a busy time for him as I recall. The truck ran OK but started stalling whenever it was in a position where I had to turn the wheel a lot while at idle to park. I called this fellow and he advised to let the computer relearn idle. I found a vacuum leak a week later and thought that was it as it seemed to be OK after I repaired it. Started to do this again on a once every 2-3 week basis, not enough to be a concern, just an annoyance. As my original post stated though, happens more often now than before. I noticed this evening that if I "let" vaccuum leak a bit, the idle rpm picks up say to 675-700 and no stalling at all when in gear or running A/C. Of course I can't run it this way, but, is it possible that this fellow screwed up and did not time the motor right (unhooking the wire etc.) and cause a "weak" idle??? I have not got a timing light yet, but would like to investigate this IF it is a possibilty. I can of course, remove the plenum to confirm any leaking but am looking for options on this if it turns out not to be the problem. Appreciate any input.

If your motor idle was OK before. Then re-timing isn't going to help. LT is correct. Replace your CPI and your troubles should go away. I have 1994 4.3 Vortec motor and it was surging at idle, but ran great at higher RPM's. Checked everything from fuel pressure to ignition. Turned out the the CPI was faulty.

probably right on that. The one thing that bugs me is that the entire distributor was replaced, not just the module, so I know it had to be changed as far as timing goes. Was wondering if a slight timing mistake would cause stalling. I am testing further, if not able to find anything else the plenum is coming off.

If your timing is off by 1 or 2 degrees. It shouldn't cause it to stall. Besides it was running fine after in distributor was installed right.

No not really. the truck ran OK but was stalling for lenghy periods due to ignition module. When the distributor was replaced it ran alight but started stalling in gear at idle from time to time the same day in fact. Mechanic said to run for a few weeks and let the computer relearn idle. I found a vacuum leak and some messy workmanship on the distrubutor (smashed wiring and loose cap) and after that is seemd to be OK but started stalling again not too long after. Random stalls, not all the time. Has gotten to be more of a pain the alst 3-4 months. Idles at 650 in park, will run all day, 600 in gear, runs OK for 10-20 sec. but then like someone shuts it off. The only other clues I have run across is the voltage seems lower than it should, (the truck's meter shows 16 volts, but my scan tool says 14-14.5 cold, 13.4-13.7 hot) and the manual calls for 14-15 plus. I got a code 35 (IAC error) but the IAC was just replaced so I do not know if that is a help, and I notice on my scan tool that the timing seems wrong. However I do not profess to being an expert on using this thing. My book says the timing should be 16 but the truck shows 20-27 most of the time in idle. Will sometimes drop to 16 for a sec or two. I had thought perhaps that the timing might be off enough to cause a weak idle. Still checking other options. I guess I need to pull the plenum to be sure of what's going on under there, but have been covered up with son's Corsica.

You said that you replaced the IAC. Did you check the insertion point were the IAC goes to make sure that you don't have any carbon build up around the ring? That could cause you to have a rough idle and stalling. As for your timing. Did you disconnect the timing bypass wire before shooting your timming? If so, your timing should be set at Zero. Then reconnect the by pass wire restart motor. Your timing base line is now set. and your CMU will be makeing the proper adjustments. Normally, it will take about 5 min.

Just a follow-up FYI, and closure for BlazerLT:

Yep, tore my intake apart and found a leaking regulator. I had replaced the entire spider and nut kit. Totally fuel tight now. I guess I was running with an off-and-on again leak until finally the regulator blew because the passenger side of the plenum was completely CLEAN. Bright, shiny aluminum. The other side was filthy. Though I had no nut-kit leak, I replaced that too, it was only $28 more from a parts warehouse in town that the local GM shops all get their parts from. I got a list-priced @ $555.90 injector kit new, factory sealed from GM in the box for $299. Upper Plenum Gasket from Napa was just under $8. Overall, a CHEAP fix for what this problem can lead to between fuel dilution, cat poisoning, EGR/LEGR siezing and O2 sensor fouling. Had to replace #4 plug because it was completely fouled out, but went ahead and sprung for #1 AC/Delco Rapidfire (non-platinum), AC/Delco original spark wires, cap and rotor too. Re-timed the engine after she was 110% rebuilt on the fuel injection system. What saved the truck again, was just parking it until I got the parts and got it together properly. Runs awesomely now!

Thanks for your input. Guess I will have to remove the plenum to be sure on this. Just cannot confirm leakage with pressure tests and looking into the hole at the top of plenum. There is a mis-fire at times when hot so either I have a goofed injector, leaky regulator, or even sticky valve. Have tried water cleaning and Seafoam cleaning with same results to clean out combustion chambers. Vacuum lines checked, replaced as needed. Old plugs showed no carbon build up, did have some white-ish residue on them but that was it.Still would like to test the pump one more time, when hot and trying to find a timing light to d/c the timing as the distributor was replaced back when this all started(also fuel pump).Both replaced same week the ignition module failed. So far nothing else has worked as far as idle goes. Will idle all day when in park, but just random-stalls in gear when the motor is hot and has any kind of load(steering while stopping, staying in gear and idleing, etc. Have been driving again and truck is still getting 20+ mpg, runs well otherwise. No gas smell in oil or anywhere else. Will advise when plenum removed as to what I find. Have a good source for CPI if that turns out to be it.

a bad cpi/nut kit will also affect engine vac. my idle was eratic and vac was only 15lbs. i changed both cpi and nut kit. idle is smooth and vac is up to 20lbs (21lbs is perfict for a gas engine).

Thanks for input. The idle seems to roam a bit and it was suggested early on that vacuum could be a problem. The idle is not constant after it warms up. Idle seems to bounce between 575-600 in gear, hot, and 625-650 in gear. Does not stay at one speed very long. These speeds may not be abnornal but it does have a stumble to it I can't figure out when hot.

Thanks for input. The idle seems to roam a bit and it was suggested early on that vacuum could be a problem. The idle is not constant after it warms up. Idle seems to bounce between 575-600 in gear, hot, and 625-650 in gear. Does not stay at one speed very long. These speeds may not be abnornal but it does have a stumble to it I can't figure out when hot.

normal idle;warm;
in park: 750-850
in gear: 550-650