excellent thank you

 

Alright I vacuum tested the fuel pressure regulator by connecting a hand-pump to the vacuum line. It does not hold vacuum. I'm assuming that means the FPR is broken since it should hold vacuum from the plenum. So I replaced the regulator diaphragm but the problem is not fixed. Could the cap be causing the issue? I don't see any damage to it, but its not stock and seems like a cheap adjustable unit.

 

Can you confirm that the car was running fine and the changes were sudden and now all of this? Thnx

 

A couple weeks before it started to buck under light throttle, and smooth out under high throttle or high load. Then it started running really rough after it was warm, to the point where it didn’t want to rev, but it would idle fine. I wasn’t driving it at this point, but had to take it out once to retrieve OBD information, while I was doing that the car died suddenly while on the road (under load) and this started. It’s been undriveable since then.

 

would you desribe your bucking as “trailer jerking” or “fishbiting”?

your issue appears to me, to be related to something electrical, and something global (affects all cylinders).

possibly, an intermittent fuel pump pressure issue but depends on how “bucking” is is occurring.

we could go through the whole fuel compression and properly timed spark thing, but i can summarize and say i think you have a spark issue.

Based on my experience (limited to 5 L98 cars and 1LT5 car) has been:

- an 87 which bucked and trailer jerked me around, then finally gave up firing at all. Took the cap off the dizzy and the metal strap from the distributor feeding the high powered electricty from the button on base of coil, to the tip of the rotor, had broken. As it was failing, spark began to get real dirty and disrupted.

- an 85, first the coil was weak and intermittent. Grounds were all checked in dizzy, it was the coil itself. replaced with another used coil fixed

- an 85 second time was the ignition control module. Which, yours could also be.

- an 89, it was either the capacitor beside the module or the pickup coil down in the dizzy stem.

- a 90, it was the ecm. looked like it had gone down on the titanic and was reinstalled in my car. It was just plain electrically unpredictable.

thats what your initial observations make me think of.



But i do have to say that the gasping in your video to me is most times indicative of minimum idle that is way way out. but it was idling fine so the min idle adjustment is probably fine. Its possible the IAC is failing or cannot keep up to the changes. Could also be the tps not set right. As mentioned earlier.

But im leaning towards ignition, and something global.

Typically when coolant temp sensors fail, they make a hard warm start condition (flooding engine because ecm is incorrectly being told its cold, because cts is fubar).

 

I would probably describe it as a jerking. It's difficult to describe but it was a sudden and repetitive lurching under low throttle. Felt like the car was just continuously jumping forward violently.

I'll check the ignition system, but it has a brand new distributor in it, as well as brand new spark plugs and MSD cables. Additionally, the FPR isn't holding any vacuum pressure and the vacuum line smells strongly of gasoline. There's also a strong smell of gas in the plenum and runners, however I haven't found any pools or anything. That's what leads me to believe its a fuel issue.
I'll check everything you mentioned though, I appreciate the input, and I'll keep the thread updated if anything new happens.

 

Lurching forward is different than trailer jerking / fishbiting. the latter is being tugged backwards.

For some reason i thought that you had already replaced the fpr with no change. apparently not! yes! thats most likely it then! that would also be global.

If its lurching forward at low speed or just off idle, then you are probably running real rich due to the leak into the plenum. the sudden and then worsening nature maybe correlative with an initial abrupt rupture and then with time that rupture got bigger.

look forward to hearing its all fixed and you are enjoying the car

 

Did you ever find the problem ? I’m having the exact same problem.