92 c1500 4.3L: stalls - won't idle - no DTC codes

[jrhalli]

Symptoms:

• Starts easily, every time.

• Idles OK for about 1 second, then dies if I don't "feather" the throttle

• When feathering the throttle, I can keep it running (under no load), but as soon as the trans is engaged or I turn the A/C on it dies.

• Again, if it dies it'll start right back up - no problem.

• When out of gear and I'm holding RPMs up to at least 1500 (and no A/C) it'll run, but the RPM is erratic. RPM will vary from 1500 to 2000 or so in a continuous pattern - as if I was revving - but my foot is holding the throttle still.
• As soon as I let my foot off, RPM's drop to about 800 (on factory dash gauge) it stumbles, sputters, and dies (and of course it'll start right back up - no problem). Almost acts like it is running out of gas.
• Once the engine is warm it seems to do a little better as far as how it runs, but even then it will not idle.
• At freeway speeds - still in the kickdown speed range - when I put my foot in it the engine acts like it wants to respond (i.e., RPM's go up, engine noise increases, etc...) but the truck is only picking up about 1 or 2 MPH every few seconds. In other words, it sounds like we're hauling ass but we're not going anywhere.

Before it got as bad as it is now I was able to drive it to and from work, but I had to put it in neutral and feather the throttle to keep it from stalling at lights and in traffic. Going back into gear and getting it up to speed was nearly impossible. If I got into the throttle too much it would bog and do nothing. But if I backed off the throttle just a little after it acts like it wants to bog, it surges and picks up power. Finding that little "window" where the engine responds is not easy though.

The truck did this once before a few months ago, and at the time I suspected bad fuel. After filling it back up with 93 octane fuel the problem was gone.

When it started this crap again I did the same thing, but this time 93 octane didn't do the trick. So, still suspecting bad fuel, I added some water remover. It didn't work. Then I added some Lucas high-octane boost and that didn't work.

I've replaced the fuel filter - no change.

Rebuilt the fuel pressure regulator with a kit - no change.

I'm thinking that it could be EGR, IAC, or TPS...BUT...the SES light has never come on and when I put the ECM into diagnostic mode there are no stored trouble codes.

With no codes, does that eliminate EGR, IAC and TPS? If the IAC or TPS are disconnected and the engine is run, would that set a code, or is the ECM smart enough to know the sensor is unplugged? Because I've done that and still get no code.

I've eyeballed the injectors at idle and under throttle and they are both squirting - both patterns look the same.

Is it still possible I have bad fuel? Or some other issue that isn't tied to the ECM? What's the easiest (in your driveway) method to check for bad fuel?

Here's the kicker...the truck is a 1992, but it only has 50,000 miles on it. It was my father's truck and he only drove it a few miles a week.

I don't think it is a spark issue because it only has 50,000 miles and it starts up fine every time...but I could be wrong.

I think I've covered everything (sorry for the long post but wanted to give plenty of info) but if I think of anything else that could help any of you with a diagnosis to help me I'll post it. Or if you have any questions I'll check back a lot and answer them.

Thanks in advance for your help.

John

[oldchevy]

How much fule pressue is being delivered?

welcome to the site.

[jrhalli]

i got a fuel pressure gauge a couple of days ago but haven't checked it yet. i'll get out there and check it and let you know.

gonna take longer than i thought...i don't have the right fitting on the FP tester...

[taillight]

Water in the gas?

[marc-o-matic]

Check the p/u coil in distributor

[oldchevy]

i was going to also suggest the pick up coil, but the only experience I have had with mine being bad was that it would not start at all. If the pu is bad, will it still start, but not run well?? I did not know that.

[2000CAYukon]

Originally Posted by oldchevy

i was going to also suggest the pick up coil, but the only experience I have had with mine being bad was that it would not start at all. If the pu is bad, will it still start, but not run well?? I did not know that.

Yes. Also check the upper dist shaft bushing for play (i.e. can you move the rotor side to side and feel play).

//2000CAYukon

[maxwedge]

Make sure both injectors are spraying.

[jrhalli]

as a test i tried bringing the RPM's up a little and as it started to stumble a buddy sprayed a little extra fuel into the TB and it picked right up....so we got the fittings and extra hose, etc for the pressure tester and after taking it through a couple of cycles the best I got was 5psi. I don't have the spec, but I think it is supposed to be around 18psi????
either way, 5psi ain't gonna cut it. only thing that still bothers me is that it made 5psi - no more, no less and it's been doing this for weeks. i won't be surprised if that 2-3 inch piece of hose on the sending unit is cracked or has a hole.

thanks for all the help guys - much appreciated. i'll post back and let you know what it ends up being.

[2000CAYukon]

TBI Fuel pressure specs are 9 to 13 PSI. 5 PSI is too low.

//2000CAYukon