With regards to my 1989 Firebird, AT, 305 TBI with 160k original miles.

For some time now, I've noticed getting periodic engine booms. It sounds like "Doomf". I notice it when say I'm stopped and in drive. You can feel it and hear it. I don't think it is a knock, as I don't hear anything that sounds like metal hitting metal. Just like a very brief engine stumble that lasts a split second here and there.

I've noticed when the Doomf is louder that the RPM needle dips a split second. And there were a few times the RPM increased a split second. I've also noticed sometimes when I'm say in line at Mcdonald's, sometimes the car will start lurching forwards and I have to push the brake pedal down as hard as I can.

The other day I took it up a hill to where it wouldn't move when in drive and with my foot off the brake. It would sit there, then I'd turn the AC on, and the car starts lurching forward. And at other times it would lurch forward for no reason.

Sometimes it sounds as if someone choked the engine for a split second. Kind of like for a split second it sounds like the engine is gonna die, but it recovers.

A couple of years ago, I was talking to a mechanic in the shop office about it, and he said he thought it was compression build up. Something about how he had a Camaro that did the same thing.

I've recently put in new spark plugs that didn't change anything. I have an open element and the filter has been changed recently as has the fuel filter. Though, I think I remember there is supposed to be an IAT sensor that goes in the filter box. I looked and didn't see it laying around the engine anywhere as I've got an open element. I don't know if that not being there might be a problem??

 

That iat is probably the thermac if there is no corresponding connector laying about. As for the doopf noise. That sounds like a backfire. Burnt valve leaking injector firing order off, there is a rash of causes for this. And the other 'lurching' symptom would be a surge. Probably a result of the backfire.

Where to begin...

More symptoms... fuel smell? constant misfire or 'shake'?

 

The last time I remember a fuel smell is when on the interstate and I did a couple of WOT's.

I don't notice a constant shake. You can barely even feel it. I just notice it every so often, and for a split second. It kinda sounds like the engine is gonna die, but for a very brief split second. If someone else was sitting in the car, they probably wouldn't even notice it.

 

Hmm... ok rules out burnt valve. Possibly mixture due to the FI looking for an intake temp... Coming up empty handed for an IAT sensor but still looking. Looks like they used a manifold air temp sensor instead... but still digging. The MAF and everything was hooked back up correct?

Edit: Looks like they just used a MAT sensor... somebody correct me if im wrong in this assessment.

 

I found the IAT. It was laying down by the distributor. Some of the plastic has melted, but I don't think that would affect anything as the metal piece is still there. But I'm gonna replace it anyways and see if I can drill a hole in my cleaner base to mount it and see if that does anything.

 

I listened to it again, and for the most part you don't hear it. Just can kinda feel it. The times you hear it is when the RPM needle dips. Like if it stumbled for just a split second.

 

you said you noticed it would happen after turning on the A/C? the a/c compressor has an electronic clutch mechanism that will cycle the compressor at certain intervals in doing so it will drag the RPM of the engine down.. if the compressor doesn't have any oil in it or is old and starting to seize. the computer will compensate by adjusting the idle speed which would be fine and dandy if the compressor was going to be on all the time but as its cycling that could produce your symptoms i would say disconnect the electricity to the A/c compressor

 

The thing I noticed with the AC was when sitting in drive on a hill with my foot off the brake. I would turn the AC on and the car would lurch forward. And sometimes with the AC off, the car would lurch forward.

What oil do I spray in the compressor?

I forgot to mention that I have the smog stuff removed if it matters.

I'm gonna also replace the spark plug wires. I know some time ago, I got a bad stumble. I took it home and noticed one of the spark plugs wires had hit the exhaust manifold and melted it exposing the wire. I taped it up and zip tied it so that wouldn't happen and that fixed the problem. So, I'll replace those to see if maybe one of the spark plugs wires was acting up.

 

Well, I replaced the cap, rotor, and spark plugs. That didn't change anything. This weekend, I plan on cleaning the IAC to see if that does anything since I only notice anything when at idle.

Do you think the O2 sensor could be causing this? I have the smog stuff and pump removed, but I still have the EGR.

 

I was looking at the black canister on the front driver side fender. There is an open line on it that says "Air". What hose is supposed to go to the "Air" on the can? Is that something for the smog? Should I close it up?

And I noticed a small round valve thing going to the black cansiter. Could that go bad and cause me some idle problems?

 

I guess what it might be is a slight stumble at idle.

Do you think it might be the EGR valve?

The EGR that is on there now is 4-5 years old. I do get a code 32 usually when I get on the interstate.

I put a vacuum on the port of the EGR solenoid that the manifold line runs to, it held vacuum fine.

There is a positive EGR on there, so I tried putting a vacuum on that, and it didn't work. I'm not to sure how to test the vacuum on a positive EGR.

I was looking around and saw the Delco EGR part number was 17113496 and saw it mentioned here that is was a negative EGR. So, should I just go ahead and replace it?

I think I've read one way to see if an EGR is leaking is to squirt some water in it to see if it leaks out the other side?

 

Well, I'm still having the same issue.

I have changed fuel filter, plugs, wires, cap, rotor, EGR, coil has been replaced a few years ago.

This may help, you can hear it out the exhaust pipe too. Dooph.........Dooph....Dooph.... It sounds kinda like a motorcycle, except there is no poping or back firing sound. Or I guess you could say an older muscle car where it sounds like it is stumbling out the exhaust pipe.