T&T Racing

The fuel damper smooths out any pulse pressure variations, I do not believe it is a factor

Chalt

I watched your videos. On Post #12, the video ... that is EXACTLY how my 86 NA is running.
Mine hadn't been sitting, it started to have a stumble, and was hard to start after sitting overnight. It would run rough for about 45-60 seconds and then settle out. After a month or so, the rough running didn't settle down anymore.
I got help from the good folks here and I tested lots of things.
I bought a $15 infrared temperature gun, ran the motor for a few minutes until warmed up, and took readings from all four header pipes while running. I put the temp gun as close as I could to each pipe, less than 6 inches to each.
I discovered that Header pipe #1 was several hundred degrees cooler than the other pipes.
I thought maybe I have a fuel injector not firing or a bad plug or a bad wire?
Couldn't prove any of those theories.
Bought a snake camera and put it down the spark plug holes.
I found out that coolant was leaking into Cylinder #1 (there was a visible puddle of coolant). Cylinder #2 looked like it had a few drops of coolant, and #3 and #4 had no leak and were dry. So, in my case, head gasket is leaking. That's why the cylinder is running cooler and rough.
I'm not suggesting I know what is wrong with your car, but it sounded so much like mine, I thought I would tell you what I found out.
I bought my camera/boroscope/endoscope on Amazon, Depstech, it was $36. I am not affiliated with Depstech. The camera works by bluetooth with my iphone and records video or still pictures and has a light on the end.

Keep investigating. Throwing parts at it is too expensive.

Otto Mechanic

Great advice Chalt. Even if that isn't the problem the OP is having, just knowing what you did and what you found is really useful.

Thanks, Interesting read. Best of luck with your head gasket!

Chalt

Thanks Scott. It was very satisfying to find out what was wrong. A mystery is great, but I like when we solve them.
-Chris

Carl Presseault

Chalt

Any news or updates Carl?

Chris_924s

Check the speed and ref sensor at the DME pinouts to verify the Speed and ref sensors are indeed good.

. Make sure signal/ pulse isnt dropping through the harness- check for Bad voltage at the DME harness plug with a MM.

Oh yeah, check your grounds. Clean and test continuity to ground at the bundles and to the DME ground- then weatherize all of them.

That's the direction I'd try first - regardless of parts already replaced.

Observe and report.

Carl Presseault

So here's the updates.

Replaced: Fuel Pressure Regulator, DME temp sender, O2 sensor and for good measure, all the spark plugs. After I replaced the O2, the car went back to life! Hooray!
.... but only for two days. This morning, it's all the same. Studder, like it run on 3 cyl. Wont stays on. I'm looking for a coolant leak now.
When it was working, everything was fine exept for two things. My oil pressure was way lower than usual. Also, randomly, I could hear a "pop" in the exhaust but not loos in power or hesitation.

Otto Mechanic

Wow. Well, if it isn't the speed/ref sensor I wouldn't have any other ideas. This would be a tough one if those signals are good. Have you been able to test them yet?

Dan Martinic

jhowell371

When my fuel pressure regulator (FPR) went bad the car was hard starting. It was also running rich. Pull the vacuum line off the FPR and check for fuel in the line. One other part you didn't replace was the O2 sensor. It fails to rich mode. Both these make for hard starting running rich. Been there done that.

jhowell371

When my fuel pressure regulator (FPR) went bad the car was hard starting. It was also running rich. Pull the vacuum line off the FPR and check for fuel in the line. One other part you didn't replace was the O2 sensor. It fails to rich mode. Both these make for hard starting running rich. Been there done that.