1987 Park Avenue, no start / kickback / backfire...

[turbokinetic]

Got a technical question, and a non-running car.

I drove my 87 Park Avenue to town yesterday without issues. Parked it and tried to re-start after about 10 minutes. It would not start and acted like the plug wires were crossed, or it was out of time. It kicked back and backfired and never started. I have replaced the cam sensor, crank sensor, and ignition module (one at a time) and there is no difference. Same firing out of time issue.

I've had these parts fail before but the symptom is not exactly like this. This is a new one for me.

If I disable the ignition so it won't kick back, it will turn over normally and sounds like it has good and even compression. I checked when I got the car about 6 years ago, and it has the new all-metal timing gears, so I don't expect the mechanical timing of the cam / crank has completely slipped.

The only electronic parts I haven't exchanged are the harness its self, the ECM, and the EPROM chip. The design of this system doesn't use the ECM for timing control until after the engine fires and runs, so I don't expect that is the problem; but if it somehow went haywire it might be causing a problem. Therefore I will try swapping the ECM with another good used one.

The engine doesn't appear to have been overhauled. It has 230,000 miles plus or minus. I am wondering if anyone has had a timing chain wear to the point it caused this problem?

I made a short video and shared to show what it's doing.

Thanks for any advice. I will post what I found, for sure.

Sincerely,
David
https://youtu.be/wUmF2hoSRdg

[95naSTA]

I would pull the cam sensor and make sure the magnet is still in the timing gear.

[turbokinetic]

I would pull the cam sensor and make sure the magnet is still in the timing gear.

Thank you. I plan to put it in the shop once I make room. Got a Coke machine, a fridge, and another car with engine out of it in the shop now.
I think the timing chain may have skipped a tooth. Engine is 230,000 or more and I don't know its history so it would be good to do the timing set. I did check (through the cam sensor hole) that it has a steel cam sprocket, when I first got the car. But as far as the condition of the chain, no telling.

[MattStrike]

My first thought on this, I believe the '87 still has the spark control module on the firewall. My recollection is that it's the interface between the PCM and ICM for the PCM to take control over spark timing, and the point at which is switches over is at 600rpm, I can look that up but it was basically immediately. It's been a long while since I've done anything with an LG3, but I think you can unplug that spark controller and it will default the timing to ICM only - which would help rule out a PCM or spark control issue and another finger pointing at a mechanical issue.

Does the LG3 have the 2-row crank sensor? I can't recall if some of them did, or if they all only had a 3x on the crank. I can dig through some of my old manuals here, my recollection was that the 3x sensor on these have 3 different sized windows, which is how the ICM knows what coil set to fire, the LG3 engines had to spin over once or twice before it would start to fire the coils - it only used that cam sensor to time the SFI. My memory may be fuzzy on that, but I would think it's possible that the balancer broke the square key and slipped to cause a timing mismatch that severe. I recall back in the day when I was farting around with my LG3 I had been able to adjust the timing on the cam one tooth in either direction and the motor still ran/started, but again it's been a long time so I don't recall exactly what I had to do to advance/retard the cam timing or if that was another engine I worked on.

*edit*
I had that first part wrong, it was the ESC controller used for KR. It is mounted on the firewall, under the black plastic shroud, next to the relays. When it fails, it causes either (1) no ignition (2) full spark retard (3) no spark retard.

[turbokinetic]

I have some follow-up on this issue. The problem was a bad timing chain set. While the original plastic cam sprocket had been replaced at sometime in the past, the metal camshaft sprocket had suffered some form of severe and abnormal wear.

The pen mark on the masking tape is where the crankshaft timing mark is located. I stacked an extra sprocket there to build up the length required for crank bolt to turn the engine with a wrench. You can see that the cam is almost two teeth advanced from where it should be.

What I find strange is how the cam sprocket is the only metal component with severe wear. The chain is barely elongated compared to the new one, and the crank sprocket is also relatively unworn. The tensioner is wasted, as you can see. I don't know if this is a result of the failure, or in some way causative. As said this engine was worked on prior to my buying the car and I have no history. There is a chance the previous repair person didn't replace the tensioner when they replaced the plastic timing sprocket, possibly.

[95naSTA]

Yikes. That is some serious wear.

[turbokinetic]

Yikes. That is some serious wear.

Yes! It looks terrible. I got very lucky that it didn't totally break and bend all the valves. I've acquired several cars which were headed to scrap because of broken chain and bent valves.

The worn chain has been affecting the engine for a while. The idle and starting are much improved. It doesn't have to be "blipped" after starting in order to stay running any longer, and the idle is very smooth.