Backfires under boost.....

Bama GN

New Member
Joined
Jun 1, 2001
My 87 GN spends a lot of time in storage. But the last few times I have gotten it out there has been a problem. Car starts great. Runs great with NO miss up to a point.

But about 70% throttle it will typically let a large backfire. Of course, that sends the system into convulsions and pulls timing. But not to worry. The car keeps on running and never misses a lick UNTIL about 70% throttle again.

Where do I start? Ignition module and coil pack? I bought a Casper's ignition tester and everything seems to be OK. I've checked every hose I can find and don't see anything wrong.

Where should I go next?
 
Mine is doing the same thing, symptoms might be occurring closer to WOT, then it runs smooth again and it only does this sometimes.
 
Bama GN -

I've been chasing an almost identical problem for a LOOOONG time now. If you do a search on here for "popping" or "misfire" you will find several threads where other members have also had a similar problem (a few of the threads were started by me). The bottom line is, there is quite a few possible causes of the symptom you described. Probably the most common is a bad coil pack, followed by a bad coil module. After that, it's starts getting harder to chase down. You said you've already checked the coil pack and module, right? If not, I recommend borrowing a known good coil pack first, then if that doesn't fix it, try a known good module. After that, a few other *possibilities* are:

- spark plugs - check condition of plugs and plug gaps (possibly gapped too wide)
- plug wires, try a set of known good wires
- fuel pressure (fuel pump going bad, clogged fuel filter, fuel regulator going bad or out of adjustment)
- crank sensor - check adjustment and connection
- cam sensor - check cap for cracks/damage on outside and inside, check to make sure Hall effect ring (cup) inside is tight (screw in middle) and not damaged
- harmonic balancer - may be cracked or Hall effect ring for crank sensor may be damaged.

Do you have anyway to determine if you're going lean (Scanmaster, etc.)? I finally purchased a PowerLogger and I've been using it recently to help diagnose the problem with my GN. All the data seems to point to the crank signal going nuts when the popping/misfire occurs. I'm in the process of checking the harmonic balancer for damage - about to post a new thread about that.

Good luck - I hope it's something simple and cheap (wish mine was!) :rolleyes:


-
 
same problem but also...

Hey, I am having the same problem, but also my car is often stalling out when sitting idling in gear. I also have the back firing problem too. I replaced the plugs, wires, coil pack, and module last summer. I have not had any codes.

What should I be looking for on my scan master, and also, we have been having a lot of rain lately. Could I be having a problem with a ground? I know the bunch of ground wires at the back of the engine can be a pain.

This has been happening only for the last 3-4 days. Also it seems to be worse when the motor is nice and warm?

Thanks brothers!
 
Check to make sure the positive battery cable isnt shorting out on the pass side exhaust manifold when the engine torques up.It will pop and usually make the dash lights flash.
 
If its popping up through the intake (watch the boost gauge spike) - say goodbye to your flat tappet camshaft.:frown:
 
No, I think it came out the ass end. When you are driving, it kind of stumbles, then surges. I was thinking that maybe i'm losing spark, then getting it back, and then unburned fuel is then igniting? At idle when it stalls, it isn't really coughing out, it is almost like someone just turned off the key.
 
little update from post #5 above...

I *think* I may have eliminated the popping/mifire in my GN. I installed a set of Magnacore race wires and the popping seems to be gone. I'm a little surprised by this since I had already replaced the wires once before and it didn't help so, not sure why it may have worked this time. The reason I say "may have worked" is because I've only had one opportunity to take the car out and test it.

THe previous spark plug wires were, first some generic wires I picked up at a local parts house (I don't even remember the brand since it was roughly 4 years ago). Then I replaced those with a set of Kirban's wires. I removed the Kirban wires and replaced them with a set of Magnacores. The popping misfire must have started after I installed the generic wires. Eventually, I thought those wires *might* be the case of the problem and that's when I ordered a set of Kirban's wires but that didn't help so I dismissed the wires as a possible cause. Then recently my good friend and turbo Buick mechanic loaned me a set of Magnacore wires. I ended up ordering my own set and they seem to have eliminated the problem.

*Also* another important symptom I was having (and this relates to the symptoms Bama GN originally posted) whenever my car would have a popping/misfire "event" it would retard the timing *and keep it retarded* until the ignition was switched off and back on again, then it would reset like everything was fine, until the next popping/misfire event.

Planning to take it out again this weekend and do some more testing.

Having said all that, it might be time to look closely at the plug wires again. What brand do you have on the car?
 
plug wires is what fixes mine. we then replaced the sparkplugs too just to be safe.
 
The wires I have on are nothing fancy. Them came from UAP, they are dark blue in colour. I bought those because they claim they are lifetime warranted. Does anyone know if the grounds clustered at the rear of the motor relate to the ignition system? I keep waiting for the weather up here to dry up, thinking that all the moisture in the air could be a problem, which I suppose can be a problem for bad ignition wires as well.... I have a scan master, but don't know a lot about it yet. How can you tell if the ignition is stuck in a retarded state, or do you need a logger for that?
 
Does anyone know if the grounds clustered at the rear of the motor relate to the ignition system?

Yes! Absolutely! If the grounds at the back of the engine are bad (loose, corroded, etc.) that will usually cause all kinds of problems, sometimes it will be intermittant. Checking those grounds is pretty high up on a long list of "things to check" when trying to diagnose ignition issues. It's not easy to get to, but you may want to spray some electronics contact cleaner on those ground bolts and then make sure they are nice and snug. Caspers makes a ground stretcher kit that relocates the ground terminal to a more accessible location so it can be periodically checked more easily.

How can you tell if the ignition is stuck in a retarded state, or do you need a logger for that?

I don't believe it was showing up on the PowerLogger or the Scanmaster. I *think* the ECM was retarding the base timing which doesn't show up on a scan tool, but I may have that all wrong. I'm sure someone will correct me if I do. ;)
 
Replacing the battery's neg ground cable fixed one for me. Had a friend that his car was doing the same thing at Bristol, turn out to be a wiped cam.
 
Bama GN -

I've been chasing an almost identical problem for a LOOOONG time now. If you do a search on here for "popping" or "misfire" you will find several threads where other members have also had a similar problem (a few of the threads were started by me). The bottom line is, there is quite a few possible causes of the symptom you described. Probably the most common is a bad coil pack, followed by a bad coil module. After that, it's starts getting harder to chase down. You said you've already checked the coil pack and module, right? If not, I recommend borrowing a known good coil pack first, then if that doesn't fix it, try a known good module. After that, a few other *possibilities* are:

- spark plugs - check condition of plugs and plug gaps (possibly gapped too wide)
- plug wires, try a set of known good wires
- fuel pressure (fuel pump going bad, clogged fuel filter, fuel regulator going bad or out of adjustment)
- crank sensor - check adjustment and connection
- cam sensor - check cap for cracks/damage on outside and inside, check to make sure Hall effect ring (cup) inside is tight (screw in middle) and not damaged
- harmonic balancer - may be cracked or Hall effect ring for crank sensor may be damaged.

Do you have anyway to determine if you're going lean (Scanmaster, etc.)? I finally purchased a PowerLogger and I've been using it recently to help diagnose the problem with my GN. All the data seems to point to the crank signal going nuts when the popping/misfire occurs. I'm in the process of checking the harmonic balancer for damage - about to post a new thread about that.

Good luck - I hope it's something simple and cheap (wish mine was!) :rolleyes:


-

my friends car did the same thing and I replaced the mass air flow sensor and it fixed the problem..
 
Top