Hi all, new to the forum but not new to wrenching. I bought a 1994 suburban this summer; well loved but not very well maintained: did alot of rehab; fluids filters tune-up etc and it was running excellent untill one day it seemed like it dropped a cylinder. Rough running, vibrating down on power etc.. I've only owned older cars without computers till now and I figured replacing the coil or distributor would probably fix it. Long story short I've re gasketed the intake manifold, replaced the egr, rebuilt the throttle body and replaced ALL of the sensors AND the computer. Checked for vacuum leaks, fuel pressure is good.

As I've worked on it, it's run better and better with the rough running returning intermittently, making the issue even more maddening as every time I do something major it seems I've fixed it, only for the issue to return 24hrs later. I'm planning on doing a compression check this weekend as it seems that it may be an issue isolated to only one cylinder? Rearmost driver's. This plug is burning clean but the last time I checked it, it seemed to have unburnt fuel on it.

I'm not sure if there could be an intermittent valve issue, that would seem very unlikely.

At this point I'm open to any suggestions

 

Could be an issue with the camshaft.

 

could also be a ground issue. as someone that started working on trucks without computers we tend to not realize the importance of grounds when computers are involved.
some other web pages that you might find useful
Obd1 code retrieval and decoding/troubleshooting

Troubleshooting sensors and how to test

 

2nd verifying grounds. Also check fuel pressure. These TBIs are very sensitive to marginal fuel pressure. I have a 93 5.7l and suffered for a very long time with drivability, rough running, stumbling. Finally checked fuel pressure. I had kept putting it off because it’s a pain and I had replaced fuel pump not too long before. I had about ~10-11 psi and it would drop a bit when throttle was pegged. Pump was an Autozone pump and I replaced it with an ACDelco based on advice from these boards. Difference was night and day. Solid 13 psi always and no drivability problems - faster starting, no rough running, etc and power I forgot it had returned.

 

As far as grounds, I know I've found some on the frame and body passenger side, two on the block and a ground near the fuel fill at the back of the truck. Would anyone know if there are any others that are easy to overlook? I actually replaced all my battery cables etc w/ some heavy Arctic ones a shop in town makes just in case it was something goofy. No change.
Where were you putting a fuel gauge in? It's mostly hardline and prefab high-pressure hose in this rig. I'd love to just install one and leave it in for quick diagnostic.

 

if you havnt take the grounds off and clean them. the one that come to mind are the battery-block and block to firewall and battery to rad support. as far as the fuel pressure reg the main location is in place of the fuel filter. but i wouldnt leave it there only for diagnosis

 

I think there is a ground on thermostat housing. Someone said that was the ECM ground but I don’t know if actually true.

here is link I followed for fuel pressure check. https://www.chevytalk.org/fusionbb/s...php?tid/253427
I bought the actron kit. Pic shows it installed @ TBI but that is hard as there is not any free play and the retaining bolt is hard to reach between firewall and engine.
I put it in temporarily after fuel filter where the retaining for hard lines is easy to reach.
Ive seen threads where people modified the lines at TBI to install a permanent gage but I think that more valuable if you are trying to tune & push performance.

Here’s a typical TBI rough running thread recently https://www.chevytalk.org/fusionbb/s...hp?tid/359442/
of course, if a single cylinder not a fuel pressure problem

 

Ended up swapping out the fuel pump, with no change. Followed up with a compression check, which I probably should have done earlier, but with the intermittent nature didn't seem as relevant... Eternally hoping it was a electronics issue..

Generally speaking the engine was luke warm, so keep that in mind, but six of eight were at 130psi, one was at 120psi and one was around 116... I suspect that these stragglers are probably to blame for the rough running, which I have to admit has become less intermittent and more full-time after the engine is warmed up.

My understanding is that if I were to put some engine oil in those low performing cylinders, if the rings were the culprit, the compression would increase, if it were the valve or the headgasket it would not.

I'm hoping this is a valve issue versus a ring issue but I am aware it may be time for a rebuild or replacement.... Could somebody confirm the oil trick so I can put myself out of my misery and decide what to do with this quagmire?

My next question is, how have people faired with Napa remanufactured motors, where I live it would be that option or pay 5k+ for one of maybe two shops to rebuild the motor and wait months on end for the return.

Thanks again for everyone's very thoughtful input.

 

although 116 seams low compared to the 130, i think the rule of thumb is as long as theyre 10% of each other. so the 10% of 130 is 117. i wouldnt blame those numbers on your engine issue

 

Just checking to be sure, did you replace fuel filter at same time as pump? Also - did you actually check fuel pressure after and what was it?

i don’t like to suggest shotgunning ideas normally but you could try replacing knock sensor as it’s not too expensive and easy to get to. A flakey knock sensor could potentially make it rough running intermittently, at least off idle, but it’s kinda a long shot. Not sure timing is adjusted due to knock counts on idle.

I wouldn’t think a what you have on compression would be that bad though no personal experience. Regardless, before I’d go down path of new engine, rebuilding I’d spend the time to get something which can datalog and look at sensors, injector pulse width, knock counts, etc while idling and see if anything changes/correlates with the times it runs rough. There’s TunerPro for a laptop which I’ve used and like. There are also turnkey units which I have no experience but many on these forums may have opinions.