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I’m in the middle of trying to track down and fix this issue, but recommendations welcome.
95 chevy 350 auto K&N stock type air filter, cheapie stainless exhaust that wasn’t complete so part leaks after cat/muffler joint, otherwise stock.
This only happens after about 20 miles of 70mph driving or longer of course.
It mostly happens when taking off from stopped, but also at slow speeds if the pedal is in the right position, and rarely at cruising speeds.
It just wants to stall unless I press the go pedal farther than this sweet spot, which is usually too much and unsafe.
I’ve noticed some weird surging or lack of power at speed, but I recently also found #3 plug wire was not on the plug all the way and fell off when I went to pull it to start a power balance test. Didn’t do the test as I was hoping this was my whole issue, it wasn’t, it was still warm and it tried to stall just getting out of the driveway. So I may need to watch for this symptom later, I may have helped that one at least.
This really all started about the time I rebuilt the TBI. If I remember right I was having hard start or no start issues before this. So I bought a rebuilt one off ebay, but it ended up not being the right one and I had to use my original lower half, I did clean it up real good though. Put on the other top half as new injectors was one of the main goals. Also put the new IACV on the original lower body.
The TPS is only 2 years old, but I have not checked it’s voltage yet. It was the main reason to reuse the original lower body, the tps mount was not the same on the rebuilt unit. I do have ground and 5 volts at the plug and 6kOhms that slowly descends to about 2kOhms when operating the throttle. So it seems it should be ok…
Plugs wires cap rotor also done 2 years and 2K miles ago, it doesn’t get driven much since it gets less than 15 mpg, but should be ok still.
I did check the EGR though as it is one of the top ‘guesses’ for this issue, all seems fine though. Pulled it, pushed it in which took some force as the nipple has a much smaller hole than the nipple would suggest, put finger on nipple and it did not move, hole to intake is wide open from what I could see.
I ran it with the egr solenoid unplugged and it threw a SES light but still did the stalling bit, so solenoid also seems ok. I though maybe internal seals were creating a vac leak when energized, it has another nipple not hooked to anything, prolly to let in atmosphere to the egr actuator. I wonder if a hand vac pump can be hooked up here to quickly test the egr actuation?
Sprayed carb cleaner all over anywhere I though vac could leak, no rpm change.
No ticks or obvious sparks on the plug wires either, checked at temp at night but not with water…
One of my worries is the fuel pressure regulator wasn’t put together correctly or just isn’t the right spring or isn’t adjusted right. So I want to check fuel pressure some time, but a little unsure how. I’ve read I need to tee in to the inlet and check there, but is there a kit I can rent?
I’ve also been researching the hell out of this issue and there are TONS off ppl writing about the same issue, but almost no fixes. gmfullsize.com forum was almost no help at all, I though for sure there would be a lot of useful info there, but it was all guesses and no real answers.
Oh and I also read it might be trans related and I looked and it was overfull so I sucked all I could out the fill tube and put some fresh back in. It does read low/cold when running, but when warm it’s the same level and I do see bubbles. But I kinda debunked this theory today as it tried to stall in park pushing throttle to the just past no throttle into the spot it usually acts up.
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I too was thinking it’s something that might be fuel related. You may need a special adapter for a fuel psi gauge to check that. You might be able to rent one at your local auto parts store. Also, it sounds like you’ve done quite a bit of work. Anytime you have issues like this and you’ve done work to a vehicle, it’s always a good idea to go back over the work to make sure it’s correct. So many issues can be caused with good intentions. You might also check out this article I wrote on solving performance issues. There may be some additional information in there that might help you.
http://www.ericthecarguy.com/faq/solving-automotive-performance-issues
Keep us posted.
I got a fuel pressure gauge and tbi adapter fitting for it. It was actually a pain to get on as the truck has steel lines running from the back of the tbi to the trans bell housing were they are bound together and bolted to one of the trans/engine studs which is hard to get to.
Once freed from this it wasn’t bad, but in the process I may have found the original issue. I found the 3 tbi mount bolts to be not very tight. I think this may have been a result of reusing the original gasket between the tbi and manifold. This would kind of explain the ‘only happens when it’s hot’ symptom and why I didn’t find it when doing the vac leak test (I didn’t test when hot).
It does however show only 10 psi when running and that will drop to like 6 at full throttle or idle, should it be more steady at 10 or more? I thought I read somewhere it should be 9 – 14 psi.
However it hasn’t stumbled off the line yet, but also haven’t done a 30+ minute interstate trip either.I think you found the problem. The fuel pressure is low. It should be 12 to 14 lbs. Six to ten PSI is too low at any throttle position.
When the engine is hot the fuel could be boiling in the fuel lines aggravating the situation.
Ok, but should I blame low psi on the new regulator or old fuel pump?
BTW I don’t think the regulator is adjustable…You would need to pinch the return line to the tank some place where is has rubber line(at the tank) or with a helper to start the engine, remove the return line at the throttle body (place rags under to soak up spilled gasoline) and plug the fitting with your finger to see if you have a pressure rise. If the increase is minimal the fuel filter is plugged or the fuel pump is tired. If the pressure rises and blows your finger off the fitting the fuel pressure regulator is bad.
It seems a little “redneck” but without the proper fittings (unless your fuel pressure tester has the correct plug)I don’t know any other way.
Actually that sounds easy enough, i’ll try it soon and report back. I hope it’s the filter, but I think it has less than 2K miles on it. I know that might not mean anything, but at least now I have a good way to find out…
Another symptom I keep forgetting to mention is bad smelling exhaust, doesn’t that usually mean too rich?
Depends on the smell. If it’s like rotten eggs it’s just the gas you’re using going through the catalytic converter which is separating out the sulfur.
As for the fuel psi issue it seems you’re onto something. You can also check the fuel psi regulator by removing the vac line while observing the pressure. It should rise when you do this. If it doesn’t, then the fuel psi regulator is bad. Low fuel psi can cause performance issues like this. It’s probably not a bad idea to replace the fuel filter if you haven’t already. That will eliminate that from the list of possible causes and is cheaper than the other fuel system parts that could be causing the problem.
Keep us posted.
I should just try a different gas station? Seems like it’s been smelly for a long time and I don’t always use the same station, but I will pay more attention the next time I use one other than the regular one I buy from.
The tbi doesn’t have a vac line on the regulator and the fuel filter was replaced fairly recently. I’d actually love to change it to a different style filter, like a screw on oil type, I really hate the factory setup, any recommendations?
Yea I forgot those TBI systems didn’t use an external regulator. Sorry about that. As for another filter I really don’t have anything for you on that one. I’ve never done anything like that on one of those vehicles.
As for the bad gas theory. I suppose it could be an issue, especially if there’s water in the fuel. If you disconnect the fuel line and run a hose into a container you can let it settle out to see if there’s any water in there. If there is, change out the fuel and recheck for the problem.
As always, keep us posted.
So I haven’t done much more work, but I did make the 40 minute drive tonight in the truck and it’s still doing it just not nearly as bad.
Will get back to it soon as try to solve this…Edit: The fuel pressure gauge never moved from 9-10 psi during the hesitations.
I started getting a CES light and ran the codes and got egr, did some digging and found one of the rubber boots that connects a plastic vac line to the egr solenoid was one I split in half to get off and apparently didn’t notice I put it back on. I hoped this was going to fix the issue, but it has not, still have the hesitation.
I installed a tee on this vac line and a gage ran up to the windshield. It was showing in the green, 18-22 inHg, when I first hooked it up, but I went out and got it warm and got it hesitating again on launch. At this point it’s showing just under the green and the gage even says late ign timing as a possible cause, but i’m also worried it’s an intake leak that only pops up after it’s hot.
Next I will try the leak detection test with brake cleaner when it’s hot and showing the symptom, but I also want to check the timing, could a bad ign module be at fault? Would it hurt to retorque the intake bolts?Another thing I noticed when it does stumble off the line is I lose all vacuum!
Would a hot cat cause more restriction than a cold one?
I do have a thermal camera I can look at the difference in temp, but i’m sure the muffler is fine as there was a piece after cat that had corroded and was open and loud and was having the stumble then too.
I don’t understand how removing the 02 will prove exhaust restriction though, is it because i’m fooling the puter or because i’ve created a hole for exhaust to escape from?
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