In porcess of pull ingmy 90's L-98 383/Superram engine because it was running very badly immediately after the rebuild. It would idle roughly, and had absolutely no accelaration. When i pulled the plugs, i noticed that the entire DS bank of plugs are carbon fouled; the PS were all normal looking, in fact look hardly used at all. I dont have a macro lens to show pics, but I'm curious if anyone could tell me what could cause these symptoms?

 

Did you verify the plug wires were not crossed up?
Did you reinstall all the o2 sensors after the rebuild?
Did you install new injectors or at least clean the old ones really good?
Carbon foul can only be due to improper timing or rich condition that I know of.
My first guess is the o2 sensor for the DS not working properly.

 

Check for adequate fuel to each cylinder, its likely that your injectors are bad. Hook up a fuel pressure gauge and note the readings, during running and after shutdown. If that pressure drops after you turn it off, they are bad. Go ahead and replace all 8 if thats the issue.

I would check out the plug wires on that side if they arent new, just to eliminate spark as a culprit. I dont believe this is it though.

You only have 1 O2 sensor on the drivers side on a 90, so the ECM should be managing each side off ot that one. If one side looks fine then this should not be the issue, though it is possible that something is up with it as well. There are testers in the catalogs to check it, if you dont have a scanner to read the data.

 

There is only one O2 sensor on this car, so I don't think that would be the cause.

What else could cause only ONE side to carbon foul?

 

Thanks guys: more info:

When i set up the engine (first time) the lash was too hard, valves werent closing, wouldnt idle. I dont know to what extent they had to be reset, as someone else fixed the lash for me, and he didn't describe the extent lash was off.
When i couldnt get it to start, i attacked the usual suspects: noid lite, ok, spark strength ok, spark distribution OK, brand new injectors, new plug wires. yes, plug wires were double checked. Correct timing was an issue. FPressure is ok, checked with gauge thru the fuel rail, and with a A piller gauge/sender.
Im just sort of mystified why one bank would be fouled and the other not.
Would an engine even run if PS cylinders werent firing? I had a very rough idle, but NO acceleration at all. I did manage to drive it home tho.
I am pulling the engine to check timing gear and some other issues that humiliated me in the rebuild.

 

Has the car been tuned?

 

I don't see a need to pull the engine right now.
Based on all you said, and the fact that one side looks good for plugs and timing is right (now), I can't belive the timing gear would be out of align. You state that each plug is firing correctly.

If the PS was not firing - the engine would run but very rough. However, the plugs would be wet. Not like new. Plus you state that you checked the firing and spark strenght (I assume for each plug).

The problem is on the DS. Either the valves are not set right, the DS head is messed up somehow (have you checked compression), or the pre-Cat on the DS is stopped up (if you have one).

 

Car has not been professionaly tuned yet. It didnt run well enough to get it to a dyno, which will happen someday soon, but there is no dyno around here. I intend to go to Finish Line Performance in Naperville at my earliest opportunity after the Gen VII is installed. The car has been modded to include a Formato chip. The spark plugs were individually checked for spark. The engine is being pulled for other reasons i wont go into right now, but while it is out I am checking the timing gear, cams and valve train and heads. When it was running, it was very rough, had to accomplish that, the distributer had to be turned quite a bit. the exhaust showed signs of being way too rich (black smoke). I agree that the DS shows this sign, i am just surprised that the problem would be so bilaterally different. Yes i agree that perhaps the PS was not running at all due perhaps to F Injector bank not working properly. that would explain these symtoms, although it passed the noid light test.

 

Sounds like you have other issues to address-why else pull the motor? BUT-I'd like to ask if you pulled the ESC wire and got your base timing set? not a direct quote-but you said you had to turn the dist. quite a bit to get it to run.Your symtoms can be attributed to improper timing-including the rough idle and carbon foulded plugs on one side.

 

Ok, Rick! Youve pinpointed my issue. Timing was NEVER satisfactorily set, and i had someone i trust try to do it. Yes, that was all done, but didnt result well. So lets assume timing was way off. ..could that be demonstrated by the spark plug pattern Im seeing? id like to think so, but dont know how it would. maybe you can elaborate...?

for instance, i like the idea that fouled on one side, clear on the other is due to one bank not firing (Finjector wiring), if it is indeed possible for an engine to idle roughly in that situation. Im checking the exhaust ports, headers, this weekend to see if i can find evidence of firing on that side.

 

This is really interesting. Let us know the outcome.

 

I'll try to explain the best I can-so-you not only have ignition timing-you have valve timing-ignition timing could be off a tooth or more at the dist. or plug wires crossed-valve timing could be related to a timing chain jumping (usually caused by strecthing over time) both can cause either pre-detonation or late detonation-usually resulting in a mix of things-like back firing either thru the intake or the exhaust-can this cause one bank of plugs to look good while the other looks carboned up? You bet!!!! or they could even get gas soaked.

I did a porting job yrs. ago on a friends Ford-put it back together thinking like a chevy guy-got #1 up and went to the dist. and put the
plug wires in-fired it up-YEP it ran-idled ruff and had no power-I tried everything-AND the lights went off-chevy rotors turn clockwise-at least
on the earlier Fords the rotor turned counter clockwise-lol-after I corrected that AND replaced those F-uped spark plugs life was good.

 

Based upon the little we really know, here is my 2 cents worth. You state that the DS plugs are all fouled (rich) and the PS looks unused (lean). Noid light shows injectors firing. You were able to get it to run, but poorly. 90 L98 has O2 sensor on DS only.

- If it was timing it should affect ALL cylinders the same. Probably not timing.
- If it was a fuel delivery/pressure issue it should also affect all cylinders the same, unless there is a restriction in the PS causing it to run lean. The O2 sensor does not know what is happening on the passenger side so it would compensate for the rough running by increasing fuel to all injectors which due to the restriction, would only take effect on the DS. Possible, but unlikely.

- The 90 L98 is BATCH fired. The ECM fires the DS or the PS all at one time and the PS appears to be ok. The batch firing for the DS is the culprit. My guess is that there is a problem with the injector open time of the DS injectors due to a ground in the circuit, a leaking injector in the DS bank, or a fault in the ECM DS bank circuit.

I'd double check the wiring and grounds from the DS injectors to the ECM. The Chevy fuel injection system supplies a constant 12v to all of the injectors. The ECM grounds the DS or PS circuits to open the injectors in the appropriate bank. An inappropriate grounding of the DS connection will cause those injectors to remain open and flood the DS intake. Since you are pulling the engine, I'd also suggest that you ensure that the new injectors all operate and flow correctly. I've seen too many brand new parts fail right out of the box to trust anything.

 

Did you try some propane around the intake?

 

Well, with a single O2 sensor it is usually mounted in the driver side manifold and those are the ones that are fouled......

 

I don't think that would be the cause though, since the ECM on this car does not control the left and right side injectors differently, right? If all injectors are being treated the same, then I don't think it would matter where the O2 sensor is located.

 

removed the headers. exhaust ports show no firing at all on nos. 4 and 8