Randomly missing cylinders

Lets here it from the HTMF gearheads…

My setup: Stock 1991 305 TBI w 5 Spd WCT5

Problem: Engine randomly runs very rough and poorly. Sometimes for days at a time. It also randomly decides to run perfectly on occasion, but usually for less than 50km. When it’s running poorly, it feels like 1 or more cylinders are missing completely, and the engine jerks and shakes violently when under load at low rpm.

My suspicions: Faulty wiring somewhere… ESCM or Coil is faulty. Or perhaps I have a valve or two sticking open on occasion?

What I’ve done: Replaced the CTS and TPS and reset the computer numerous times. Checked each plug wire for spark while idling, which showed me that each plug is firing.

Timing is set to 0*. I re-timed the car this afternoon after replacing the IAC connector. I have adjusted the timing from -1* to +4* while the engine is running rough, and the problem persists. This issue doesn’t appear to be related to the timing or advance, because with the EST bypass unplugged, the engine still runs poorly and misses, with no or very little change in performance.

~ Just to note: After I re_timed the engine this afternoon, I took the car for a drive around town. The engine was running very poorly. I’m convinced at times it’s down one or more cylinders, and at a steady cruise you can feel it jerk as if it wants to go, but can’t. This leads me to believe the problem lies in the ignition. Unless valves can stick open just sometimes… Anyhow, the whole drive was fairly rough on me, the clutch and I’m sure the engine aswell. Here’s the weird part… Less than 2 blocks away from my house, the engine note suddenly changed, and the car ran smootly. I gave it some gas and I was happily thrown back in my seat. She runs fine right now, but I know tomorrow or later on tonight it’ll crap out again.

What could possibly be causing this sporatic misses?

That is the problem with engines, one problem can be a million things wrong. If you have the time to take it off the road, check the valves and when it is running poorly, ensure all the leads are firing (ensure it is during poor performance). Might also be poor rings or shot gasket. Ensure your plugs are clean and firing.
We had a similar problem with the jaguar, it was a combination of timing and fouled plugs because it is meant to be a touring car.

It could be a number of things why the engine is not running well.

The guess of the valve(s) sticking could be one of the possible reasons why the engine is not running the way it should.

Ok so here are some other reason why its not running the way it should:

  1. the O2 sensors are totaly shot or way off. but if this is the reason the check engine light should come on.

  2. Not sure of the ignition type but this is another possiblty. The vacum advance on the distributor might be out of wack. or just a vacum leak some where.

  3. the egr vavle might be stuck if the motor has one.

  4. some thing wrong with the throttle body or the injectors.

  5. something wrong with the pistons example: The rings are shot and you barely have any compression.

to bad i dont know to much about those motors.

Hope this helped.

-Kyle Nelson

Thanks Kyle.

When the O2 sensor is disconnected, the SES light comes on and also goes out when plugged back in. I figure the sensor is fine, it should throw a light anyway. It seems every other sensor in the car has thrown one at one time or another.

The car doesn’t have vacuum advance, and I replaced all vacuum lines to the EVAP and such, so there shouldn’t be any vacuum leaks either. I can’t hear any when it’s running. I unplugged the EST bypass (advance control) and it wouldn’t advance the timing at higher RPM, so that still works too.

EGR isn’t an issue.

As for the TBI nozzles and functioning… They emit a very clear and unique sound and when the TBI opens up, they appear to spray perfectly evenly and there is no oscillation or abnormality to the sound or appearance of the nozzle spray. The TPS increased fuel flow according to position aswell. I don’t know how to physicaly test them, is there something else I can do? Fuel filter is clean and new.

The compression… I started to do a dry test today and couldn’t find any tape to mark the wires, so my lazy ass gave up for the day :wink:. First cylinder had a nice clean plug in it, without buildup, the gap was still correct and 155PSI dry. I can’t find my Haynes, but i think it should be around 170lbs. Probably to be expected for a 270K engine, comsuming oil. I’ll finish up tomorrow… A wet/dry compression test should show if the valves are sticking in any cylinders without pulling off the valvecovers, unless the rings are really, really shot. Oil consumption is up by the way.

Corrupt, all plugs fire and each pulled wire drops the RPM. I’ll need to wait and see if all the plugs are clean burning during the compression test.

You might need to drop the cylinders out to get a good handle on them. I don’t know that engine well at all, but you might be able to get a semi good look if its just got a oil sump below you can drop to get a veiw from the crankshaft up the cylinder. That engine is a V8 so that might be a little time consuming, but quick that taking the cams off and the heads off the cylinders. If you don’t need to screw around with cams, don’t, thats my experience.

It’s just annoying that the ECM doesn’t recognize and throw more errors. I’ll build an ALDL cable tomorrrow to see what kind of information my ECM gives out. I think between the ADLD scan and compression test, I’ll get a fairly solid idea of whats going wrong. If compression and ALDL say stuff is OK, then the problem must lie in the ESCM (Electronic spark control module) or coil. Both are about $50 bucks from NAPA, and in stock here in Kelowna.

Any suggestions, thoughts & commenbts are welcome. I need this thing alive to get back to Rupert.

Just a thought there, Goose… if you can start your car up after several hours of sitting, so it’s cold, and fire it up, and it’s misbehaving… check the exhaust from side to side of the engine… it might give you some ideas to check if one entire bank of the engine isn’t firing.

Do you have one or two catalytic converters (or any, heh)… and if so, are there any oxygen sensors downstream from the cat(s)?

We had a V8 Dakota at work one clogged cat, and it would run extremely rough from time to time, and we noticed that half the exhauset wasn’t heating up much at all.

Well, he says all the leads are sparking, they only reason for non-ignition of 4 cylinders would be lack of fuel (although I don’t know the fuel injection system on these Firebirds) there is a chance of there being a problem with the injectors somewhere the fuel pump but before the fuel line branches to the individual injectors for each cylinder. That’s just a shot in the dark, because a '91 Firebirds fuel injection is a lot different to the '67 Jag (had 3 carbs and not FI) and the '97 Civic I worked on.

I’m almost certain the fuel system is fine. I use fuel system cleaner to ever second tank, be it STP, MotorMaster or RedLine. The fuel filter is 15,000km or so old, and still looks clean. Anyway, the car is still running great. She’s got another 50km since I last posted. If this keeps up tomorrow after my compression test, I’ll advance it back to +4/+6 degrees and see how she runs. Most L03s crave 6-8* advance with 89-91 octane.

Any knowledge of spark control modules and such and how to go about getting them tested, if possible? I’d hate to buy one and have nothing change.

As I wrote in thirdgenorg…

“Without the air cleaner on, the injectors appear to spray evenly at all RPM levels. The TBI has a very distinct sound when operating, and there isn’t any abnormality to the sound. IE: intermitent spray or air pockets… etc etc. How would I go about ‘checking’ them, other than watching them do their thing?”

OH! And Eso, temperature seems to have absolutely nothig in common with this problem. Sometimes it acts up when warm, sometimes when cold. So far, it’s totally random.

Wait overnight till the engine cools and do what Steve said, start it and feel both exhaust pipes, if one gets warm before the other, it could be a loss of cylinder(s). Quite an interesting situation you have here.

As a part of what eso said to do, i just had my cat removed and the rest of my exhaust replaced. I had what sounds to be the exact same problem with my truck, when i gunned it sometimes it would just hesitate and spudder a bit. At first i thought i was the fuel system, but that was all clean and good. Then i thought it was my lifters, but since i couldnt hear any ticking, i assumed they were fine. Then on an impulse, i wanted to get the exhaust fixed since it was making sick leaking noises. Since i has it replaced, the problem has yet to re-occur. The guy the the exhaust place said that my cat was completely shot, and they to make your engine run like shit when shot.