'91 L98 Auto, 48K miles, bought 6 weeks ago, have been battling this issue since a week after getting the car. Have spent many hours on these forums searching for solutions and believe I have eliminated all but three possible causes but any help would be appreciated.

When accelerating above 20-30 MPH the motor starts to bog and will surge the more pedal pressure. Under light acceleration its subtle but putting your foot in it much more than an inch results in RPM drop, some surging, and once heard a thunk. Idling in gear is rough (shakes car, some flickering of instrument lighting), idling in park or neutral is smooth. Sometimes under moderate acceleration there is a throaty rumble that is not normal exhaust sound. Does not stall in any condition. Starts beautifully. Highest I've taken the car is 45 doing this because I'm concerned I'll cause additional issues by attempting a highway run.

I am new to working on vehicles beyond oil changes, brakes, etc., so you will not be surprised I went into the parts spiral. To date I have replaced: IAC, O2 sensor, throttle body gasket, transmission fluid, oil change, EGR valve, fuel filter, TPS, MAP, spark plugs/wires, fuel pump. Yes, thats a good bit of cash I've thrown at it but its a hobby vehicle and the contract with my wife is I get a pass as long as I'm doing the work and the costs aren't egregious. As far as I can tell the car is bone stock minus a tint, alternator, and aftermarket wheels from previous owner.

I am down to three possible issues that I have come up with and any thoughts would be helpful.
- Vacuum leak. Have checked the low-tech way but without a shop I could easily be missing one
- Fuel Injector leaking/cracked/failed. I am very hesitant to dig into the fuel rail and once I do I'll have to send them off since there isn't a super dependable local shop that can work on old injectors
- Grounding issue. I've gotten to the grounds I can find and cleaned them but likely missing one somewhere.

I don't *think* its a transmission issue because its fine under 30 and I'm still able to get it going into what I think is 3rd gear. Yeah, I'd know for sure in a manual but I my feet are too wide to operate 3 pedals in a C4.

Any ideas? Any more information that would help? I'm reaching the point where I feel I need to get professional help (as best as I can in a semi-remote area of North Dakota) so I can enjoy the car for a while. Thanks!

P.S. When I turn on the Radio (bose, cassette) I get a very loud tone and nothing else. I'd like to keep the original radio (or at least a C4 radio with cassette) so in anyone's experience is that a control module, head unit, or amp issue?

 

It does sound like a fueling issue.

Have you checked the fuel pressure?

 

Check / replace your fuel filter first.

 

* have not yet, will do that thank you

* replaced a couple weeks ago, have you ever seen a situation where a new filter could get clogged tgat quickly?

 

Just in case... pull the vac line off of the FPR and sniff for a fuel smell.

 

No, not that quickly…didn’t know you just changed it.

 

If your exhaust system is blocked up with disintegrated pre-cat chunks it could cause this type of behavior. Mine did...

If the pre-cats just behind the manifolds break apart, all the pieces travel back to the main center cat and block up the entrance. Enough of it and it builds massive backpressure that prevents the engine from revving above a certain point and makes everything funky.

 

That is absolutely wild. Did you notice overheating or a lot more heat towards the rear (radiating from rear wheel wells) before you cleaned that? Took it for a leisurely drive around the neighborhood and after noticed the car was quite warm but stayed in the 200-210 engine temp range. Thank you for the picture!

 

Thank you to everyone for the help and suggestions! Shes back on the road!
After 6 weeks of shipping delays, work trips, and a week of actually getting the job done, finally got the injectors replaced. Runs incredibly smoothly and super happy to have my car back. Once I actually tested the injectors it was a slam dunk on problem ID...only 3 were reading in the right range and one was at 6 so had almost completely failed. The team at injector connection got me a great product but couldn't help tough shipping issues here in N.D.

I did have a shop take care of clogged but not blocked catalytic converter, which wasnt the problem but doesnt hurt anything in the end.

Now all thats really wrong is a check engine light that sometimes comes on when cruising at highway speeds but when I put the OBD on it gives the standard 1-2 code. No noted performance issues or anything different, just the light comes on then is off next start.