I have posted before about this issue. I don't mean to frustrate anybody with reopening something that has already been discussed, but it has been so long since I've thought about this I am treating it like a new problem again. I haven't touched the car in months, largely because I got so frustrated with this thing, but also because I haven't had much time or money for it. Its a 1991 6spd.

Background
This began with a misfire under light loads, that would clear under heavy load (WOT, low gear, going up a hill), and would not present itself at idle or revving in neutral. I determined that the sending unit had rusted and clogged the fuel injectors (the baskets were visibly blocked, and I found debris piled up in the fuel rail). I replaced the sending unit, injectors, and filter, and cleaned the fuel lines with brakeclean and an air compressor. After all this the symptoms remained the same, but the misfire was now significantly less pronounced. It has gotten worse over time since then. I would describe the miss as a violet jerking / bucking and you can hear the exhaust cutting out rapidly.

Already Done
Spark plugs seem to be fine from a visual inspection. They are fairly new, no deposits or cracking or anything. The plug wires are new, and I tested it with old ones too with no effect. The ignition coil is working properly, and again I tested some old ones with no effect. From tunerpro, all sensor readings appear to be in order. Fuel pressure is fine when in neutral, didn't test while driving because my gauge is kind of short.

Moving Forward
At the moment I have theorized that I did not clean the fuel lines adequately and the new injectors got clogged too. But before I go about fixing that I want to be sure, I do not have money to waste "fixing" something that isn't broken. Please, if you have any ideas, I would like to be more confident in my diagnosis before I start fixing. Any ideas are greatly appreciated.

 

Maybe check your timing.

 

I'll do that. I'll do my best to get it done after work tomorrow but if not I'll get to it on Sunday.

 

One possible way to do a gross check on the injectors is to pressurize the system and, while observing the pressure gauge, manually activate one injector with a 12V source and see how fast the pressure drops.

Repressurize and move on to the next injector.

It's an extremely imprecise test, but if one injector is really clogged, you may be able to detect a difference in how fast the system depressurizes.

Other than that, check your cap and rotor carefully... look for signs of carbon build up, cracks, etc. Your original post didn't mention these things so not sure what you've done there.

Also, post some tunerpro data... I'm pretty familiar with the 90-91 EFI systems. I'll be happy to take a look.

 

I kind of had the same issue with my 92. I made a post called Turbo Vette to describe the problem. It felt fine accelerating under normal load, but when I got on it it would lag at low rpm and then kick in above 3k rpm kind of like turbo lag, pretty much stock LT1-no power adders. I got a laptop, aldl cable and tunerpro and data logged it. It was pulling a lot of timing below 3k and then behaved at higher rpm. I ended up getting another chip on the forum here and it improved it a lot, but I still feel it needs to be dialed in. My car is also a 6 speed. Had over 180k miles when I got it. Also had a flowmaster cat back exhaust so I suspect it has been modded by who knows.

 

Timing is set at 8 degrees before. Thanks for all the replies guys, I have to get to work right now but will post some tunerpro data and check the cap, rotor, and injector balance when I get back.

 

The other thing to check is TDC on #1 vs the mark on the balancer. If you get a piston stop, you can determine #1 TDC exactly at the piston, then look at the damper and verify the timing mark is at 0 on the marker. There have been instances of the outer ring on the balancer slipping, and that can throw off your timing measurement.

 

Ok I removed the cap, there were some light deposits on all the contacts but I don't think it was anything concerning. I cleaned it up with some fine sandpaper. I underestimated how tired I would be right now, and its too late to start the car (out of courtesy to neighbors as the car is very loud), so I haven't yet done anything else.

I did create a little harness with a button for testing the injectors, and made a little piston stop. But I will plan to continue with this tomorrow.

Here is an old bin I created the last time I worked on this thing. Back in November you (ULTM8Z) guided me in creating this bin. Here's the thread: https://www.corvetteforum.com/forums...misfire-3.html
Because we went through the computer back then I am looking more towards physical problems, but if you see anything or think of anything I'm happy to revisit it.
I originally attached a bin / log from November but have deleted it because I do not know what experiment was being run when they were created and am unsure what the car is actually running on right now. I will produce a current bin tomorrow.

 

I would confirm the wiring at injector connectors are good, I had an annoying misfire on my 85 TPI. Turned out some wiring was damaged at the injector crimp connection, also had one connector strip had come apart making a poor contact.
Had me stumped for quite some time, I narrowed it down by starting the car from cold and shutting it of after a few seconds. Then felt each header pipe warm, warm and one was cold !. Swapped the injector connector with the one next to it, as they batch fire. and transferred the fault to next cylinder, replaced all the injector electrical connectors problem solved 🙂👍 So worth a look, as a lot just tug the electrical wires not connector and if just a strand of wire makes contact won't provide enough current to run injector continually. Good luck 👍 Fuel lack of it is way more noticeable, than a bad spark plug or lead. Will really "shake the engine" if an injector is not firing properly.
Should be something simple, narrowing it down to a particular cylinder helps. Also check each spark plug, look for any odd color or deposits that would indicate the cylinder is not firing like the others. 👍
Also check the compression, in case you have a burnt or bent valve 🤔
I had a bad new camshaft once, would start idle then start to shudder and die. Would not restart had redone the valve lash again, same started then would run rough and stall. Was the damn base circle of camshaft had 10 to 20 thou irregularity would pump up lifters and loose compression..... Now that was a pain in *** to find, but saying it cause it goes to show some things are almost impossible to find. Someone mentioning check compression before you run the car and after it stalls, is how I found it. An old timer who had come across this issue, his tip was gold!
While it won't be your problem, its worth thinking outside the box try other things 👍

 

Lots of great advice here! Just a note of caution: if you don't have much experience using a piston stop, Please be extra careful. Best advice is to remove all other plugs to relieve the extra compression force of each cylinder. It can be rather difficult in a healthy engine to decipher if your just rolling thru a compression stroke on a diff cylinder or coming up against your stopper. I learned this the hard way on a hi-compression build years ago and had to replace the rod bearings on that cylinder, as it forced past and deformed the bearings. Bent the bass stopper instead of a piston skirt (forged ) but lessons def learned! Good luck Sheriff 👍

 

A bad Throttle position sensor can cause weird behavior like this also. Maybe try a fresh fuel filter before doing more expen$ive work. If the system was that dirty, it may just need a few filters to catch & clear everything out. Once the lines are cleared, the pump-sock in the tank and the inline filter should handle all the rest. Vacuum leaks can cause some of this part throttle behavior too. Just my 2-cents 🙂

 

Went back through that older thread. Seems like we left off with what I'm suggesting this time too... that injector test.

At the beginning of all of this, it was running good until you changed injectors?

Also, what O2 sensor are you running? Bosch? Those have been known to be problematic sometimes.. they kinda get a bad rap on the Thirdgen.org boards...

 

Yep. Sure can. Just replaced the TPS on my '90 last week. Same symptoms. Well, that and the timing was a bit retarded, too.

Swapped out the TPS and advanced the timing to stock 6 degrees and all is well now.

Also installed eight new Bosch III injectors, too, as I had one dead one that didnlt even read on the multimeter.

Still had the symptoms, even after the new injectors.

New TPS and advancing the timing to stock is what solved it in the end.

Though having all eight injectors working helped a bit, I imagine.

 

thanks for all the replies. I did have trouble using the piston stop so I instead just stuck a screwdriver in the spark plug hole so I could feel the piston moving. The timing mark on the balancer coincided very nicely with the piston reaching the top and going back down. So it seems to be correct.

I also did the injector balance test. I started each one from 39PSI and used a metronome to get the timing as accurate as possible. All injectors dropped very uniformly, none of them were noticeably different.

The 02 I'm using is a GM brand: I converted it to a 3 wire to help with tuning a few years ago.

I have replaced the fuel filter and pump sock. I should have mentioned that in the beginning, I guess it slipped my mind. I did this both when I cleaned the lines the first time, and just a couple days ago. It made no difference.

 

I checked the injector wiring, it all seems to be fine. I did have to replace a couple of them a few years ago, but the others seem in good condition. Moving them around while the engine is running doesn't have an effect. I have tried to figure out which cylinder is misfiring by measuring the head of the exhaust tubes, but it won't miss at idle, or even at any RPM in neutral. And when I drive it, it either spends enough time not missing or gets heatsoaked enough that there isn't any real difference in temp that I can tell. The miss also usually doesn't present, especially not at full force, until the engine has warmed somewhat.

 

About September or October is when this began, with the same symptoms I'm having now but MUCH more violent. It happened very suddenly too. Bad miss at moderate load, goes away under heavy load, not present at idle or in neutral. The car jerked and bucked so much it was basically undriveable. I eventually found out that the fuel rail was full of debris and the injectors were visibly clogged. That is when I replaced the injectors. (as well as filter and sock, and cleaned the fuel lines). When I assembled with new injectors, the problem was still present (though not as severe). This is when I made the original post.

So, the main idea is, the problem began without me doing anything. It began missing while I was driving the car, I didn't replace or "fix" anything until after it began.

I think I can pretty safely say that the original problem was caused by clogged injectors. I will attach photos I took of the state I found the rail and injectors in. Keep in mind the pictured injectors are NOT on the car anymore.



At this time there is no visible debris in any injectors or the rail, although when I was replacing the filter the other day I did notice the line leaving the filter seemed to still be dirty.

 

I have replaced the filter and sock, no results. The TPS is maybe 1.5 years old. I plugged in to tunerpro and it is outputting sensible data. The connector and wires are all in good condition.


Thanks again for all the replies. I tried to address everything here, but if I missed something let me know. I can't do a compression test at this moment but I can get to it sometime in the next couple days.

 

Compression tested good.

 

Ok. Ordinarily I'd say do a leak down test, but what that test is designed for would probably more noticeable at idle speeds where things are slow enough for a leak down failure to manifest as a miss. That happened to me several years ago when I had a mysterious misfire at idle that I finally had traced to the #7 cylinder, which then failed a leak down test, ultimately traced to a stuck piston ring, which I got un-stuck with some oil additive thakfully. Above idle, the misfire would disappear since the higher engine speeds meant that there wasn't enough time to bleed off compression.

You're saying the miss isn't present at idle, so I think the compression test is what would be the most perceptive in that regard. The fact that it goes away at WOT suggests that it's a closed loop fueling issue (at WOT the mixture typically goes rich and the ECM doesn't pull fuel for any reason). It also (fortunately) suggests nothing is seriously mechanically wrong with the engine and that something is just fooling the ECM into pulling fuel out in closed loop.

Yeah, this one is a mystery to be sure.

What about the things in posts 4, 7 and 12 that I suggested?

 

Post 4 -> I did the injector balance test, and there was no real noticeable difference between injectors. I also inspected the cap and rotor. There was some small deposit buildup which I cleaned but nothing concerning or that would cause a problem.

Post 7 -> I verified TDC, the mark is accurate to the #1 piston movement.

Post 12 -> I did the injector thing. My O2 sensor is a GM brand 3-wire

Back in November I created a bin that forced open loop all the time. I also created a bin that added a lot of fuel under all conditions. Neither of these changed the problem at all. It's also important to mention, the misfire clears not only under WOT but under heavy load as well (in low gear, with moderate throttle but not necessarily wide open.). Because of this I don't really think it's a computer problem, although if you have tests for me to run, or things for me to check I will very gladly do so.

I think I may redo the compression test. I was a little rushed the first time. I don't think I'll be able to do anymore today however.