Insane missfire ***PLEASE HELP***

TurboWh1

TurboWh1

Well-Known Member
Joined
Aug 12, 2002
Hey guys!!

I've been chasing a problem on my 86 t-type. I've got a missfire at around 2400rpms. But the weird thing is you must be on the highway under a load to get it to do it. It will not miss just holding the engine at 2400 rpms. It actually backfires out the exhaust and looses all power until you let off of the gas. As soon as you do the engine returns to normal and may not act up again. This is a very interminant problem.You have to drive about 15 miles to get it to misfire.

So there is the story and here is what I have tried to do to correct the problem. I've swapped the ecm,chip,coil and module,crank and cam sensors,mass air reads correct,and I've checked the continuity on the wires that lead to these sensors/ecm. I feel like I'm chasing my tail here and I really want to drive the car to BG this year!! Have any of you guys ever experianced this problem?? Any advice would be appreciated!!
Thanks,
Brandon
 
Have you checked the valve springs or the cam? Sounds like there is fuel building in the exhaust.
 
Does the check engine light come on or flash? Have you checked your fuel pressure? More so after it acts up, if the fuel pump is getting weak or hot when your driving, low fuel pressure could cause some drivability issues/mis-fires.
 
A bad TPS sensor caused my car to backfire. See if you have a drop out under load. You won't find it with key on/engine off like when you set it originally.
 
Check plug gap. I know my gnat will knock them out spec. after some time. I should check them now, I'm feeling a slight hesitation at high speeds.
 
TurboWh1 said:
Hey guys!!

I've been chasing a problem on my 86 t-type. I've got a missfire at around 2400rpms. But the weird thing is you must be on the highway under a load to get it to do it. It will not miss just holding the engine at 2400 rpms. It actually backfires out the exhaust and looses all power until you let off of the gas. As soon as you do the engine returns to normal and may not act up again. This is a very interminant problem.You have to drive about 15 miles to get it to misfire.

So there is the story and here is what I have tried to do to correct the problem. I've swapped the ecm,chip,coil and module,crank and cam sensors,mass air reads correct,and I've checked the continuity on the wires that lead to these sensors/ecm. I feel like I'm chasing my tail here and I really want to drive the car to BG this year!! Have any of you guys ever experianced this problem?? Any advice would be appreciated!!
Thanks,
Brandon
Click to expand...

Brandon,

I don't have much in the way of suggestions to help, but I can tell you you are *not* alone with this type of problem. Do a search on this board for either "popping" or "misfire" and you will find a lot of threads on the subject. Sometimes it's as simple as a bad ignition coil, sometimes it's a mechanical problem in the valve-train. I've been chasing a similar problem for over 3 years now and *still* haven't figured it out. My most recent thread on it is here: http://www.turbobuick.com/forums/scan-tool-tech/277648-need-help-interpreting-powerlogger-data.html

I purchased a PowerLogger and it's a great tool for this type of diagnosis. It might be time for you to purchase or borrow one. Although I haven't nailed down the problem yet, the PowerLogger is helping me eliminate causes. I just don't have much time to work on it.

I've replaced about 95% of the ignition and fuel system so when I finally do find and fix the problem, this car should run like a top!

Keep after it and keep posting your progress. Good luck!

Rob
 
First, high speed data logging like direct scan or the powerlogger is invaluable in tracking something like this down, so here's your excuse to get one if you don't already have it. If nothing shows up then that's kind of indirect evidence it's ignition related since that won't set an error code. Second, key on engine not running use a voltmeter or scan tool to watch the tps as you very, very slowly move it from wherever it is at no load/2400 rpm to past highway load/2400 rpm to see if you have a bad spot. Third, it's a real long shot but I saw this with my own eyes on one car - check the cam sensor setting and once it is "right" try moving it slightly each way (had a friend's car whose chain must have been stretching just enough to move the cam sensor to the next window under certain load/rpm conditions). Turn the sensor so the wires move about 1/8" or so. Fourth, see if you can get it to do this by slowly bringing the rpms up against the converter, like you are building boost on the starting line. That way you can troubleshoot in the driveway :). Last, I fought something like this for a year on my car, intermittent pop and stumble at 72-74 mph. The second new ignition module and coilpack finally fixed it, after lots and lots of checking and replacing of other stuff.
 
Hey Guys! Thanks for all of the replies!!

I havn't checked the valve train. I will have to look into it. I rebuilt the engine and it only has aprox 4000 mi since the rebuild. I know that doesn't mean anything. But I did visually inspect the plugs and they did look ok. But I will just replace them anyways to see if it helps.

The check engine light does not come on or flash when it starts backfiring. I have watched the fuel pressure. It stays solid within 1psi when it is backfiring. However the one time I actually got the engine to stall it did drop to aprox 33-34psi.

I was kinda thinking about the tps. I have a scanmaster. I will try driving it on that setting to see if I get any varriance.

Thanks again!!
Brandon
 
Solved the problem guys!!!! I had a hairline crack in the harmonic balancer and the bolt backed off!!:eek: She's back to running great again. I've put about 600miles on her in the last two weeks.
Thanks guys for all the advice!!
Brandon
 
Congradulations Brandon! I think I'm finally getting close to having my problem diagnosed also (see post #8) and I'm now thinking my problem may also be related to harmonic balancer. Planning to check it tonight.

So did you replace your balancer or just tighten the bolt? If you replaced it, what did you replace it with?

Rob
 
I replaced the balancer with a stocker and retorqued the bolt. I also added some locktight. The car runs smother than ever. I almost wonder if the balancer had been cracked all along and I just never noticed it. Or the torque wrench was bad that I used. One of the two. I'm glad to be driving the car again. I had 3 people stop me yesterday to try to buy the car!! It really made me feel like all the hard work finally paid off!!:biggrin:
 
TurboWh1 said:
I replaced the balancer with a stocker and retorqued the bolt. I also added some locktight. The car runs smother than ever. I almost wonder if the balancer had been cracked all along and I just never noticed it. Or the torque wrench was bad that I used. One of the two. I'm glad to be driving the car again. I had 3 people stop me yesterday to try to buy the car!! It really made me feel like all the hard work finally paid off!!:biggrin:
Click to expand...

Excellent! :biggrin:

I tried to remove my harmonic balancer tonight but the bolt is DEFINATELY not loose! I snapped a #3 Craftsman screw driver trying to stop the crank from turning while using an 18" long, 1/2" drive breaker bar to try to turn the balancer bolt. (started a thread in the "Engine Tech" section about this) What is the torque spec. on the balancer bolt? Mine seems over tightened.

Thanks,
Rob
 
Did car have a missfire all the time or just after car had reached running temperature. I'm having the same problem. My problem comes after car has reached running temp. I've changed plugs, wires, ign module, coil, fuel pump, filter and car still doing same thing. plan on checking that balancer though..
 
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