1988 Chevy TBi - Mystery Misfire

Hello and greetings from Germany. I have a problem with my '88 GMC Van with a 305 TBi engine. I have a dead miss on one cylinder at idle - confirmed by cylinder drop test, vacuum gauge shows fast fluctuation 19-21 inHg, and the spark plug is significantly blacker than the others.

Problem is I cannot find any reason for it!

Swapped the spark plugs and leads around but in any case now the engine has new plugs, wires and a new distributor. Compression and 100psi leakdown test showed no big issues, the "bad" cyclinder had aroud 25% leakage, another one only 15-20% but. A scope on the injectors showed a regular square wave, no gaps. I double checked the firing order and I checked the valve clearances and watched the valves opening for the correct plug position on the distributor and with the cylinder at TDC. Even took the intake off to check for blockages.

I have been trying to do ignition waveforms and I get a decent one off the coil king lead, but the problem is I cannot get a trigger of one of the plug leads. I am using Hantek 1008c with the HT25 pickups. On the plug lead (tried multiple differnt) the waveform is a complete mess, it sometimes shows a waveform but not consistently to get a trigger. I tried also using my PeakTech 4250 current clamp as a pickup on a plug wire and this just caused random peaks in the waveform, nothing I could get a trigger from.

I got a waveform anyway covering all 8 cyclinders at warm idle (attached) and I don't see any obvious problem with one cyclinder anyway, but can't do a snap throttle test easily without knowing which cylinder i'm looking at.

Any ideas appreciated!

You can ground one of the other plug wires to get a sync. Pick one that isn't close to the bad cylinder in the firing order.

If you're confident that the spark to the misfiring cylinder is solid, and there are no vacuum leaks on the runner to the same cylinder, then I'd try a running compression test.

Compare to another known good cylinder (or two) for reference. A difference in running compression represents a difference in cylinder breathing, which can account for your misfire.

You also should not have a square wave on the injector it should start at battery voltage the drop straight down to 0 volts and hold there for a few millisec. Then spike upwards to 20-50+ volts then slope back down to battery voltage like a kids slide with a bump (pintle closing hump) almost at battery voltage. If its a square wave you have a problem there. Post pic of the 2 injectors waveforms please.

Also this could be a case of some weak valvesprings but lets just go down the list of what everyone listed.

Is this a constant miss over a wider rpm range or only at idle? (25% Druckverlust ist ein schon hoher Wert, 25% pressure drop at 100 psi is more than normal)

The sampling rate of the scope combined with the selected screen time does not allow for an analysis of the ignition wave form......

Hello everyone and thankyou for all the useful input so far. In reply:

The cyclinder in question is #5 sharing the intake runner with #8. Off this runner is the main vacuum intake for the cruise, accumulator etc (closer to #8 than #5) but I smoke tested it and didn't see anything obvious. Can do it again to be sure. #8 seemed to be fine with a drop test, plug condition etc.

I did a compression test a while ago and it was around 130 psi I believe with a couple of other cylinders at 150 psi but I will re-do one on a few different cylinders, I will try this.

The injector waveform is a good point - this was done with the injector connectors off and just cranking as they are pretty sealed up and I don't see an obvious way to backprobe them. Will try and work out a way to a get a waveform pattern with them running.

I can do a leakdown test again with a couple more cylinders. I agree 25% could be significant compared to 15% on cylinder 7, I was just going off it was still in the green area of the gauge.

I really like the idea of shorting another plug to ground to see which one of the waveforms is the missing cvlinder, but I'm not sure it helps getting me any further in terms of focusing in on the missing cylinder waveform to get it in a decent amount of detail and test what it does at high rpm/snap throttle etc. It's really frustrating I can't get a decent trigger signal of the plug lead, as I assumed this must be possible as they use plug lead pickups for timing lights etc. I can try my setup on another distributor vehicle to see if it shows anything up.

eumecon wrote: I really like the idea of shorting another plug to ground to see which one of the waveforms is the missing cvlinder, but I'm not sure it helps getting me any further in terms of focusing in on the missing cylinder waveform to get it in a decent amount of detail and test what it does at high rpm/snap throttle etc. It's really frustrating I can't get a decent trigger signal of the plug lead, as I assumed this must be possible as they use plug lead pickups for timing lights etc. I can try my setup on another distributor vehicle to see if it shows anything up.

If you've got a timing light you could try opening it up and 'scoping its trigger signal. Or an automotive DMMs RPM pickup also works.

www.scannerdanner.com/forum/lab-scope-di...ico-2205a.html#42534

Does the engine oil smell like gas. Ie pull dipstick out and smell it. If it does this might be the injector that handles that runner leaking or maybe the valves just need adjusted/ cleaned. Do you have a video scope/ camera scope to look down into the runners to the back side of valve? Not sure but a cracked valve or valve seat can cause that leakdown percent. As for the dead miss did you ohm the wire leads and check inside the sparkplug boot to see if the electrode wire is crimped under the metal clip terminal for plug? Sometimes they need pushed on more with the boots and they feel like they are on. Use some dielectric grease on the wire side of spark plug and same on the cap side of plug wire and see if it changes with plug out and clipped to a metal part of engine. Does it spark then?

Thanks for your inputs so far. Took me a while to get back to looking at this again, did around 1500 miles in the meantime with zero issues.- Pulled the plugs on the rear four cylinders, checked the spark plug condition and then compresison tested all 4.

Pressures were

Cyl 5 (dead miss) and 7 (ok): 205psi
Cyl 6 and 8 (both ok): 180psi

The plugs all also look fine, far left is number 5, slight browning but others also showed evidence of this. Previously the number 5 looked a little wet, but this was after a lot of idle as opposed to now after a lot of motorway miles.



- The suggestion of using a timing light pickup was great, I realised I actually had a clamp like that for an old multimeter and it works perfectly, can now get the seondary waveform with a trigger! No idea why the amp clamp didn't show anything similar. This is waveform at idle



Comparing against the next cyclinder I don't see anything too extreme



And under snap throttle



I also checked and double checked the plug wire and it is on securely and the plug definitely fires outside the cylinder. So it seems I have ruled most things out, again! Thoughts on the waveforms appreciated....

u could have a worn cam lobe on tht cylinder remove valve cover and check

Hello guys,

I'm not too expert at catching misfires through secondary ignition wave forms, but according to Paul's book, that seems you have a lean condition (not only in a single cylinder).

I'm in the process of adapting a complete TBI chevy system to a 1970 Toyota FJ40. To set this thing up (without scanner), I had to perform many old school tricks that can be useful to rule a lean condition out.

Just to enrich the mixture a little bit, you can add a resistor in series with ECT sensor (let's say 330 ohms for this little lean condition you are facing). This won't hurt anything for your test.

I think this test can give any direction in order you have proven a lot of things and you still don't know the root cause of misfire.

You can get same results by increasing fuel pressure. This can be done by adding force to the spring inside pressure regulator or by strangling a little bit the fuel return line.