1991 P30 454 intermittent surging, power loss

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Hi everyone,
I just bought this 91 Damon Challenger P30 454 TBI and it has this intermittent issue at slower speeds. It doesn't do it all the time.

It idles fine, has trouble accelerating, power loss, massive engine speed surging while driving, and an occasional backfire through the intake. If I shut the engine down for 10 minutes, it may run perfectly fine. Air filter looks new, and both injectors are spraying.

Strange issue, I am stumped. Please help.

[enblethen]

It could be a vacuum leak. Cracked hose or a bad emission valve. Check the hose going to the EGR solenoid and the solenoid to the EGR valve. Possible bad or stuck EGR valve

[Stim]

I don't think it has been mentioned but, next time it is warmed up and acting up try removing the gas cap and listen for a vacuum sound.
That would point to an emissions problem not venting.
Another wild shot, possible muffler partially blocked.
If it was a Ford you would be spending money on suspension corrections.

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I really appreciate everyone’s help, but for right now my time has run out for troubleshooting this. I have to leave with it on a trip tonight for a festival, and I will be gone until after Labor Day. I will have to resume troubleshooting when I get back in September.

When I get back, I will learn how to pull the codes and post them here.

Thank you again for everyone that has contributed, and try to help me.

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Gjac wrote:
You say it bogs down when accelerating at low speeds, what does it do at high speeds? It this problem only at low speed? when engine is hot? Does it run better when engine is cool? Does it run fine once you get above a certain speed? Have you checked the timing yet? Base timing should be 4 degrees BTDC, 454's run better when timing is advanced to 9 degrees. It almost sounds like you're not getting advance when you accelerate if your problem is ignition or you're not getting fuel flow from the fuel pump if it is a fuel issue. Your pressure could be fine but if you don't have the correct flow rate, it can cause hesitation. There is a spec for this, x number of units per min. If you pump is original think about how many gals it had to pump at 8 mpg verses your car at 30 mpg over 58k miles.

There seems to be no rhyme or reason, sometimes it will accelerate perfectly fine and it doesn’t matter if the engine is hot or cold. The problem is just at low speed, and accelerating from a full stop. Once it gets up to high speed, it doesn’t seem to have a problem going down the road.

[rgatijnet1]

You have changed a lot of parts but it is also necessary to clear the fault codes that may be in your ECM. Unless you clear them there is a good chance that the ECM is still responding to the old faults.

[Gjac]

You say it bogs down when accelerating at low speeds, what does it do at high speeds? It this problem only at low speed? when engine is hot? Does it run better when engine is cool? Does it run fine once you get above a certain speed? Have you checked the timing yet? Base timing should be 4 degrees BTDC, 454's run better when timing is advanced to 9 degrees. It almost sounds like you're not getting advance when you accelerate if your problem is ignition or you're not getting fuel flow from the fuel pump if it is a fuel issue. Your pressure could be fine but if you don't have the correct flow rate, it can cause hesitation. There is a spec for this, x number of units per min. If you pump is original think about how many gals it had to pump at 8 mpg verses your car at 30 mpg over 58k miles.

[udidwht]

enblethen wrote:
I would not source GM distributor. There are many after-market rebuilt that are better.
If you are going that much work, I would suggest after-market coil and new Lifetime plug wires.
udidwht: 58,000 is not even broken in! Have over 200,000 with no major engine work.

Aftermarket these days equates to China = **** or ****-shoot. If you have 200K in one of these Class A's with a 454 that is good but also an exception rather than the norm.

Definitely pull the codes and at min check fuel pressure and look for vacuum leaks.

[enblethen]

You need to check for codes!

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**New update**

I rebuilt the TBI throttle body. I really did not find anything wrong with it, but it's done.

Bought a new A1 Cardone distributor. It has the same reluctor design as the original GM. It also has two holes with screens on the base plate, presumably to address the moisture issue inside the distributor. I like this updated design. The new distributor was easy to install with no problem.

Spark plugs were Autolites, I replaced them with AC/Delco.

Spark plug wires were the original AC/Delco. One of them had a large split in it, near cylinder #1. I installed a new set of AC/Delco wires.

**Test drove, no change. Problem is still present.** It bogs down when accelerating at low speeds.**

I tried disconnecting the MAP sensor and guess what, It runs much better!!

Ordered a new AC/Delco MAP sensor, installed it and test drove. Ran good, but only for 15 minutes and the problem is right back.

Disconnected the new MAP sensor, and it runs much better! The hesitation is still there a little bit, but it runs significantly better with the MAP disconnected.

Every time I disconnect the MAP, I plug the vacuum port. FYI

I still haven't looked in to pulling codes on this, but the CEL (Service Engine Soon) light comes on immediatley when the MAP is disconnected and turns off when it is reconnected.

I am starting to wonder if the PCM is bad.

I should have bought a FORD! This thing is very frustrating.

[enblethen]

I would not source GM distributor. There are many after-market rebuilt that are better.
If you are going that much work, I would suggest after-market coil and new Lifetime plug wires.
udidwht: 58,000 is not even broken in! Have over 200,000 with no major engine work.

[udidwht]

If you replace the distributor be sure and source a GM one. All others are a **** shoot in terms of quality. The cracked reluctor can certainly be a cause for your issue. But be sure and verify your fuel pressure or you'll be chasing your tail. 1991 fuel pressure is 9-13psi.

These 454s love plug wires. Heat wears them out. What I have done is buy 'Heat shield' cloth cut to 8-10in lengths (8 of them). And slide one on each wire before placing onto the plug. I used 2000F rated. Has worked very well. Also purchase some HD ceramic ended wires.

Pull the code/s using a paperclip inserted into ports a-b of the OBD connector. Search Youtube for a video of how to and reading of flashes.

Once you get the code/s post them here.

[enblethen]

On HEI distributors there is a wire that controls the spark advance. If I remember right, it is an individual wire going into the distributor. Make sure it is connected.
This takes the place of vacuum advance system.

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**Update**
I removed the fuel pressure regulator. Nothing was visually wrong with it. I am going to proceed with a TBI rebuild kit from NAPA.

I decided to remove the distributor cap. Cap and rotor have seen better days.

The reluctor (Star wheel) is cracked in 3 places! And it is rusty. I think the rust is common in these units. I may look at replacing the entire distributor.

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enblethen wrote:
Did you change the fuel filter?

Yes, I installed a new Wix fuel filter.