Backfires & pops @ wot

[Matt McClung]

My 84 gn pops & backfires at wot, I have plenty of fuel pressure, I also notice that it hesitates a lot off idle then all of a sudden, it will pull like mad. I just took off the timing cover and the marks were right where they should be at TDC. Any ideas??

Thank You
Matthew McClung

 

[crazygn]

plug wires burnt, coil pack going bad.

 

[Matt McClung]

I just noticed that although the chain and sprocket look good, the chain tensioner is missing and the cam button is the old plastic one with a weak spring. would not having the chain tensioner cause a problem? And if so where is a good place to purchase one? Oh BTW, thank you for the reply Crazy GN, the coilpack and wires are new.

Thank You
Matthew McClung

 

[crazygn]

i dont think it would cause it to back fire but maybe cause it to jump the teeth with out it. not sure where to get one but the parts wanted. Is the cam sensor in good , not 180 out or any thing right.

 

[crazygn]

if you just put it back together then maybe check your fuel injector harness were they plug into the injector, this happened to me before and back fired because one was lose.

sorry trying any thing

 

[Matt McClung]

okay, here is the story, the car runs fine unless you give it full throttle. But, when the car is cold after you first start it, you can give it all the throttle you want. As soon as it warms up, it will backfire at wot. So even though the coil is about a year old, could that be the problem? maybe the ignition module? I have had the front cover off and checked the timing chain (new double roller), and it looks fine. Also reset the cam sensor. This car starts right up and idles well, no hesitation, runs great at ALL rpm's when cold, but backfires at WOT when warmed up. Maybe Crank sensor? Also has great fuel pressure.

Help please!!!!

Matthew McClung

 

[crazygn]

did you change your o2 sensor, maybe its starting when the o2 sensor starts to read.

i'll think about it and get back, gotta go eat.

 

[Matt McClung]

I also noticed that the crank sensor for spark has been hit from the flywheel at one time or another. even the bracket that holds the sensor has a crack in it. Maybe the sensor is good down low in the RPM's but can't keep up when revved up?

Just speculation but the sensor has been hit for sure.

Thanks
Matthew McClung

 

[BLACK Ttype]

Coilpack getting heatsoaked possibly?

 

[Matt McClung]

Hey guys, I just put a timing light on the car just for the heck of it and my spark timing is all over the place. it hits the mark sometimes but it travels back and forth. Would this be the fault of the crank sensor (which has been hit and is cracked), or the coilpack, or ignition module?


Thanks
Matthew McClung

 

[cool 84]

The computer controls the idle using timing. The IAC does the big adjustments and the fine adjustments are done with timing so it's normal for it to jump all over the place.

As for the timing chain, you don't need nor should you run a tensioner with the double roller so you're ok there.

It could be any number of things like already mentioned. I had a problem like yours once when the bolt for the front pulley was loose. It would idle and run ok but as soon as I got on it, the crank would thrust forward and since the pulley was already forward more than normal it would pop and backfire due to the loss of signal. Most likely not the problem but it's possible. Could very well be the coilpack, plug wires, plugs, injector harness, temp sensor, ground problem when warm, etc. Another similar problem I had years ago was the cam sensor was too close to "the edge" When I hit high rpms, it would scare the crap out of me from the backfiring. I loosened the bolt and bumped it slightly and it solved the problem.

You could run the rpms up with the shifter in first gear with very little load to determine if it's load based or rpm based.

 

[Matt McClung]

Thanks for the reply, it is load based, like you said I can run through first gear to 5500 rpms without issue. but when I am going a fair amount of speed and just stand on it, the boost guage goes up to around 15 psi and pops, but when the engine starts to rev up, then the pressure will drop to around 7-8 psi.

Thanks
Matt McClung

 

[Matt McClung]

ttt

 

[cool 84]

Being that it's load based I think that rules out the cam sensor and crank sensor. What are your plugs gapped at? I had to close mine down to .030 to cure a misfire above 16psi. What're the 02's like?

 

[Matt McClung]

I'll take a look at the plugs and as far as the o2 sensor I don't know since I don't have a scan tool. I took a drive and messed around a bit, and it will start backfiring at about 10 psi. Anything below that it is fine, I was just thinking, is it possible that I might have a weak intake valve spring and the higher the psi, the more that a valve could float and cause a backfire?

Just a thought.

Thanks
Matt McClung

 

[Matt McClung]

ttt

 

[Turtle TR]

maybe a bad cam sensor? i had the same problem before...i changed out my cam sensor with a friends one and cured the problem.

 

[cool 84]

I still say plugs, wires, coilpack, or a lack of fuel. If it fires cleanly at high rpm but not under load I doubt it's crank or cam sensor.