1999 Blazer stumble/stall

Ok.. I've been having a problem with my Blazer. It's a 99 4.3l 4x4 with automatic.. The problem is totally intermittent, I can't seem to find anything in common with any occurrence, other than the engine is at full temp; it has never done it cold.

Out of nowhere, it will start acting like it's running out of gas, rpm's will jump around all over the place. And just because you give it more gas, doesn't mean the rpm's will go up, sometimes giving it gas makes it worse. Sometimes the rpm's will get low enough that it will just stall out. But it cranks right back up, and the problem continues. I've heard a lot of people with this problem have their dash lights go out- that's not happening to me, all my dash lights are fine. The voltmeter does not fluctuate at all during this, so I'd think that would eliminate the charging system. It happens idling, or while driving at any speed. Usually it wont be bad enough to actually stall out unless I'm idling or at a very low speed. MOST of the time it runs like a champ, idle speed right about 600 rpm. The fuel pump appears to be fine, I can hear it whining for the 2-3 seconds when I first turn the key, and while the car is running. My gas gauge is screwy, which I assume is the sending unit, which is a very common problem for the blazers, but I wouldn't think the problems are connected. I'm not getting an SES light, and it does come on when I first turn the key, so I assume the light itself is fine - however, to test that theory, I unplugged the MAF sensor, but the SES light did not come on. Maybe that wasn't a good enough "test". I checked to make sure the gas cap was on, as I know that can cause screwy problems. It will do it whether the A/C is on or not. If I turn the truck off and let it sit for a while (as little as 10-15 minutes), the problem will dissapear until the engine's been running again for a good 20-30 minutes or so. Again, this is an intermittent problem which makes it that much more confusing.. Like 90% of the time it runs great, but I don't want to be in the wrong place at the wrong time when it stalls one day, nor do I want the problem to possibly progress to something worse.

I don't have the money to go to a mechanic and have them parts swap or whatever mess they're going to do - I can do the work myself, if I can just get help diagnosing the problem, I'm trying to avoid an endless barrage of guessing with new parts until I get it right. I've seen that this seems to be a fairly common problem, but without a fairly common cause.. ? I've gotten several suggestions, including the following:

fuel pump and/or filter
ignition switch
ignition module
ECU
EGR valve
throttle position sensor
02 sensor

Does anyone have any other theories, or can help me possibly eliminate any of these?
I'm sorry post is so long, I wanted to be as detailed as possible - if there's any other info that would help diagnose please ask.

Thanks so much

Chris

P.s. I have another, seemingly less important problem - I hear a squeeking noise that seems to be coming from the front wheels, which is consistent with vehicle speed. The faster I go, the more often it will "squeek". Does this sound like brakes or wheels?

How many miles on tune up stuff?
How is that fuel filter?
Two things to do to start with.
Check for stored codes present and history.
Does check engine light come on when it acts up?
Run a fuel pump pressure test on it.
And tape a fuel pressure gauge to outside mirror or outside of windshield and watch fuel pressure when it does it.
Guide lines on fuel pressure test.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak.
Post back fuel pressure readings.

That's half the problem is I just bought the truck, and don't really have any records for it to know what/when stuff was done. I'm planning on doing plugs/wires/cap/rotor/fuel filter here soon. checked PCV valve and it's fine. just changed oil and filter, and air filter, all last week. No SES light when it acts up. I will take some pressure readings as soon as I can get some cash for a gauge :banghead:

As MT said check those things and add the idle air control (IAC) and crank position sensor to your list of possible items. The IAC sets the idle (on throttle body) and can get all crapped up or have loose connectors - clean the throttle body and air passages. Then that CPS sometimes also acts strange and never sets a code. If the ignition checks out and all new stuff and still happens that CPS may be it - but step thru the othres first.

Check for loose wire connectors on the ignition system, i.e. Coil, Distributor, Crankshaft Sensor, Camshaft Sensor, VCM connections, and all the wire connectors between these components. Make sure the little plastic clips are not broken off and are actually clipped to hold the connectors together.

I had this same problem and found the 12V wire connector to the coil was not seated all the way, just hanging there.

well now the SES light has come on, and stays on.. I don't know if it's to do with this problem or not, but I'm going to take it in for a tuneup (since I need to anyway), and I'll get them to hook the scanner up, and also check fuel pressure for me while they're at it. Just out of curiousity, anyone know around the going rate at the GM dealer for a full tuneup?
plugs,wires,cap,rotor,pcv,fuel filter? I can get that done at Speedee for about $150, but I trust the dealer and their parts more.. just don't wanna pay out the butt. Normally I'd do it myself but I don't have the time right now :/

Do not go for a low ball price at a cut rate place.
A good repair is going to cost you money.
A good full tune up with cap and wires and everything can run over 200$ for parts alone.

Tips on finding a good repair shop.
Check around Ask around
Word of mouth.
Family friends coworkers neighbors business people and delivery people.
Mailmen and parcel delivery people get around and notice a lot of stuff.
Even a good parts house knows what shops are good and not good.
Chamber of commerce and better business bureau and city hall.
If you find a good referral to a repair shop go look them over and talk to them.
Look for a clean looking busy place with nice people running it.
Ask a few questions and ask about their qualifications and training.
Not all places have trained tech/mechanics.
If they do not find a place that does.
Even all dealers do not have all trained techs/mechanics.

Good Luck and let us know how it goes.
MT

code its throwing is P1870, transmission slipping. which I noticed it doing the other day, slamming into gear from 1st to 2nd. But it's not doing it anymore, at least not that I can tell. Could this be causing all my problems? And what does this most likely mean? Torque converter? Will post fuel pressure readings soon

P1870 is a transmission problem code.
Fix the engine running problem first.
A engine has to be running good for the transmission to run right.
Now are you taking it to the shop for repair or you going to do it yourself?

ok.. did the fuel pressure.. at key on engine off, it immediately jumps to about 65-66psi while the fuel pump does it's 3 second deal, then holds steady at 63 psi. once started (cold start), it goes to about 56 psi and stays there. let it idle for a few minutes, no change, stays at 56. I gave it some gas and it jumped to 60 psi, let off the gas and it dips down to about 50, quickly jumping back to around 56. turned ignition off and it went back to 63 psi. I had thought i was going to do repairs myself, but if it's the fuel pump, like i'm suspecting y'all are going to tell me now with these fuel pressure results, then I'll let someone else do it. I'm a little wary about dealing with the gas tank, plus it should (should!) be covered under warranty. is the fuel pump what it sounds like to y'all?

Fuel pump specs are good. Remember that the pressure regulator is vac dependent and under the upper intake cover so as you blip the throttle the vac changes and so does the fuel line pressure.

Dealer charges lots for their plug install and more for the wire install.

ok.. did the fuel pressure.. at key on engine off, it immediately jumps to about 65-66psi while the fuel pump does it's 3 second deal, then holds steady at 63 psi. once started (cold start), it goes to about 56 psi and stays there. let it idle for a few minutes, no change, stays at 56. I gave it some gas and it jumped to 60 psi, let off the gas and it dips down to about 50, quickly jumping back to around 56. turned ignition off and it went back to 63 psi. I had thought i was going to do repairs myself, but if it's the fuel pump, like i'm suspecting y'all are going to tell me now with these fuel pressure results, then I'll let someone else do it. I'm a little wary about dealing with the gas tank, plus it should (should!) be covered under warranty. is the fuel pump what it sounds like to y'all?

On the fuel pressure test you need more testing.
Fuel pressure specs are 60/66 on your engine.
Key on pressure is good.
But we need the engine cranking pressure.
Your engine running pressure is dropping below specs.

Engine running should range about 61-62 full vacuum no load or idle.
A throttle snap or a load on engine no vacuum the pressure should go to 65-66 range.
Steady driving down the road or light throttle it should range 63 -64.

But you need to get a fuel pressure gauge taped to the outside of windshield and drive it 30 minutes to a hr and watch the fuel pressure after the engine and fuel pump has warmed up good.
And also watch the fuel pressure when it acts up.
Like catching it in the act of acting up.

From what you have checked your fuel pressure is dropping down after engine is started.
But that does not always say the fuel pump is bad.
The fuel pressure regulator controls pressure as long as the fuel pump has pressure to control.
You need to check the direct fuel pressure at the fuel filter to check pump.
It should put out 75-85 lbs of direct pressure to be good.
Also a pinch the return line of will give you a fuel pump or fuel pressure regulator check. which should also be done after fuel pump has been run 30 minutes or more.
So before you replace the fuel pump do the full testing on it.
You could have a fuel pressure regulator problem or other problems besides the pump.
MT

ok.. more thorough testing:

key on engine off = 64psi
cranking= about 68 psi
idling = 56-57psi, stayed the same after idling about 5 minutes
driving = about 60 psi at 2k rpm cruising.. about 65 at WOT on interstate.. taking foot completely off gas, but still moving, about 56-57 psi. I drove for a good 45 minutes and checked these numbers several times, stayed the same the whole trip
idling after driving = 56psi
turn car off = jumps to about 64psi, then slowly creeps down. took a little over a minute to go from 62 to 60, after 2 min. it was at 55psi, after 5 min. it was 47psi

I tried the pinching return line, but all the fuel lines I could see were metal. there are two going into the throttle body, both metal, one having the test port on it. I followed both of them down the engine compartment as far as I could, and traced them underneath the car all the way back to the filter and a little beyond. nothing was "pinch-able".. ?

I also (of course) couldn't duplicate the problem to see what the pressure did when it happens. Whenever I actually WANT it to do it, it won't :screwy: . But it seems to me like the fuel pressure is fine except at lower rpms (which is when the problem tends to occur, btw). Once I get below about 2k rpms, the psi starts dropping below 60..

From the prssure readings it looks like the fuel pressure regulator is a little off.
Specs are 60/66.
At full vacuum or engine idle or letting up on gas when driving you should see 60-61 lbs fuel pressure and on a load or eng pull at low egine vacuum the regulator should raise fue presure to 65-66 lbs.
Light eng load should range 63-64.
The fuel regulator is controled by engine vacuum.

A good pinch off place is between engine and frame on the rubber part of fuel lines.
Or check fuel pump direct pressure on the end of fuel line where it goes into filter.

Your pressure is a little low at full vacuum or idle but it will run all day in the 55-60 pressure range.
Witout acting up.
Unless your fuel pressure is droping below that when it is acting up you have a ign or pcm or wiring or sensor problem besides fuel pump.

How often is it acting up?
If it is not doing it 2-3 times a day it is going to be hard to find.
I would suggest you go ahead and do the tune up and find some one with a good engine/transmission capable scanner to test it out for you.
A good engine capable scanner will show present or history problems and codes and data.
Good luck and let us know how it goes.
MT

It only happens maybe 2 or 3 times a week, I'm thinking mainly because I use it mostly for short trips, to and from work, etc. And it only seems to do it when the engine's been running for a while (30+ min. or so). Also, I'm thinking it only does it when it's fairly hot outside, 85+ degrees. We've had a cold spell the past week or so and it hasn't done it at all in that time. Engine's not overheating, but I know that heat and electrical stuff don't get along. Should I really just not worry about it unless it becomes a more frequent problem? It's mainly an annoyance at this point, because, just my luck, it almost always happens when I'm in bumper-to-bumper traffic, and it's maid me stall out in the middle of that traffic several times. But it cranks right back up.. I dunno!