(this message is re-posted from the TPI board)

89 Firebird, 355 TPI - I've been trying to diagnose an afterfire situation for a couple weeks now. Initially it was so bad I couldn't hit the gas without dumping a half gallon down my headers and having it all explode at speed. I replaced the knock sensor and have calmed this down somewhat, but the car is still basically useless on the road as I am still losing power and having my fuel explode in my pipes. Below is what I have checked and the result:

-fuel pressure ok
-injectors all ohmed 16 cold, 17 hot
-battery/MSD connections solid
-plug wires not burned through/ohmed ok (MSD 8.5's)
-plugs look almost perfect
-valve springs ok/not broken
-no exhaust leaks (that weren't there before this started)
-O2 sensor connections ok, sensor working correctly
-knock sensor ohmed infinite resistance, replaced with new (scanner shows normal knock counts, with little to no timing retard when "it happens")
-coolant temp sensor ohmed cold (4.72) and hot .25 - ok
-TPS was a tad low, adjusted to .49
-ignition coil ohmed ok both cold (.5/5.4) & hot (.5/7.0) (Blaster 2 GM) on both primary/secondary
-distributor cap & rotor looked ok
-ignition module tested ok (Advance Auto)
-pickup coil ohmed 830 (could not see to check for breakage)
-base timing was 8*

So all this checked out ok, what more can I look for? Can I test my MSD box? Also, I'm not sure what compound I used under the Ig Module last time, should I use Dielectric grease, RTV blue, or the silicone grease that was unused from the module when it was new?

Something I thought peculiar, when i had the gun on the timing mark, I noticed it would "spike" or jump (to about 15* or so) every couple of seconds. For those who will ask, the EST was disconnected. Also, Vader had suggested I disconnect the EST when my problem crops up and see what happens. I did and it was still happening..somewhat. It did calm down a bit but I could still feel a bit of fire in the pipes, but it wasn't dropping the RPM and actually popping.

I don't wanna fight this for another 9 months like I did on that stupid pulsator issue I had last time. Please point me in the right direction!!!

Thank you...
Eric

 

anyone please?

 

Srry cant help on the backfireing thing, never owned a FI car. With my carb it is usuraly related to timing. You shure it is not detonating?

Use Dielectric grease on the moduel.

 

do not use dielectric grease you should be using a thermal compound supplied with the module... it gets very hot and needs to dissipate heat to the base of the distributer and needs to be able to ground to the dist base

 

It seems I may have fixed the car by accident. I found my knock sensor was dead, so it was replaced initially. That helped some. One of the last things I checked was the ignition module & pickup coil. The coil ohmed out fine and the IM checked out ok too. I did notice that there was almost no grease under the module. Funny thing here, as I was digging around in my tool box, I found the little pouch of Delco heat sink compound upopened. Hmm....I remembered that I used dielectric grease under the module (from advice I got in one of these forums last year). I used the entire pouch of compound and bolted my dizzy back in. Note the timing mentioned above; I dialed the base timing back a couple degrees and viola! It runs & idles like a champ now. I took it out for an hour and flogged the crap out of it without so much as a hiccup.

I'm glad that's over in time for t-top season!