The Caprice acts EXACTLY like it has a burnt plug wire or a bad ignition module, (dead cylinder BIG time) and pops through the carb unless the timing is WAY retarded. (makes me think crossed plug wire, or 180 out distributor.)

It has fairly new plugs and wires (gonna change those tomorrow just in case)
It has a brand new ignition module.
None of the plug wires are crossed.
AFAIK the actual cam/lifter/rocker/valve of the offending dead cylinder(s) are good MAJOR PITA to check.

Why is it doing this? I get 15 MPG on the highway cause I have to keep it FLOORED to keep up with traffic.

I may try a new ESC (EST?) if I can score one cheap (dumb me forgot my two extra in Ohio...)

87 Caprice, divorced HEI, LG4.

 

Maybe a clogged cat.
Is it carbureted. Make sure the choke is adjusted properly.

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2 Tickets to game= $250
Gas= $20
Sitting 20ft. from MJ= Priceless
'86 IROC T-TOPS, TINTED WINDOWS, BRAKE LIGHT BLACKOUTS
GM GOODWRENCH 350
EDELBROCK HEADERS Hooker CatBack
EDELBROCK 600CFM CARB.
KN AIRFILTER
ACCEL HEI DISTRIBUTOR
160* Stat, just switched to 180* b/c of winter coming and going to college in the mts.
3:73 Posi Rebuilt 700R4
B&M Megashifter, 5" Autometer Tach w/shift lite

[This message has been edited by Mark A Shields (edited November 25, 2001).]

 

The cat was gutted before I bought the car.

Choke works fine

 

Compression check?

Look for 2 adjacent cylinders with low readings, especially #5 & #7....

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"So many Mustangs, so little time..."
ICON Motorsports

 

Yeah, start with a compression check and go from there. Backfiring through the carb makes me think it's a valve-related problem, however, more than a head gasket. It could also be a cam lobe(s) going flat.

 

timing chain skiped a tooth on you.

 

i had a similiar problem when my 305 was in my car. At cruise it was fine, but start pushing the accelerator down it would pop (not backfire) through the carb. Turns out it was the plug wires crossfiring. Double check your wires again, and make sure none are touching (especially 5 and 7). I've had it happen more than once that the plug wires were the problem, but the cheapy replacements were STILL no good.

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85 Berlinetta Iroc exterior NOW 454 powered, 4.10 gears, 275/60/15 Hoosier Quicktimes freshly installed th350 3200 stall, shift kit, reworked valvebody, Megashifter, Biondo linelock ..........YES, it's street driven!

check it out at http://www.bsronline.net

 

That's exactly how it acts, and is STILL running like crap with new wires. Haven't put on new plugs yet. Old ones have like 1000 miles on hem.

I inspected the rockers too, they're all moving and oiling fine. I'm at a complete loss.

Thinking about the timing chain now..

 

BTW, I don't have a compression tester, and money is REAL tight right now..

I'm about ready to just hand the bastard to a mechanic. ugghhh..

 

BTTT

 

Timing chain theory doesn't make sense. If I understand Mac right, the problem is ONE cylinder. Timing chain will mess 'em all up pretty equally.

That said, the only things I know of that can cause a bad cylinder...

Bad plug, bad wire, bad post in the distributor cap (seen it happen), wiped cam lobe(s), or stuck valve(s).

The popping through the carb is strange though. How did you verify that it is only one dead cylinder anyway?

 

Which cylinder did you isolate as the dead one? Is the popping consistant on every firing stroke? Or only when thew load is increased on the engine?If it's only under load, i would still say take a look at your plug and wire on the offending cylinder. A crossfire will produce this when cylinder pressure causes enough resisttance that the spark can't just the plug gap. The spark will try to find the path of least resistance and if 2 el cheapo pulg wires happen to be lying together, voila it will fire that cylinder instead because more than likely it is not on the compression stroke, thus the cylinder pressure is less then the proper firing cylinder. Usually this will happen with 5 and 7 because they are right beside each other on the same bank, and follow each other in the firing order. If a crossfire occurs, the cylinder that is NOT supposed to fire is on the intake stroke. With the intake vavle open, and the plug firing, there's your pop through the carb. That could also explain why it will stop when you retard the timing.
If the pop happens consistantly, I'd look for a burnt,stuck valve. for whatever reason, on the firing stroke the the flame front is reverting back-up the intake tract.

If the pop is constant but very erratic, take a look at your cap and rotor for aany signs of carbon tracking. You could have a spark scatter problem within the cap, but i highly doubt this.

Good luck

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85 Berlinetta Iroc exterior NOW 454 powered, 4.10 gears, 275/60/15 Hoosier Quicktimes freshly installed th350 3200 stall, shift kit, reworked valvebody, Megashifter, Biondo linelock ..........YES, it's street driven!

check it out at http://www.bsronline.net

 

It only happens under load, which is why I doubt it's a valve/cam/lifter.

 

then I'd have to say double and TRIPLE check everything on your secondary ignition. Even put a pile of wire looms in to keep those suckers apart. Just out of curiousity, what kind of replacement wires did you put in? are you running an aftermarket coil?

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85 Berlinetta Iroc exterior NOW 454 powered, 4.10 gears, 275/60/15 Hoosier Quicktimes freshly installed th350 3200 stall, shift kit, reworked valvebody, Megashifter, Biondo linelock ..........YES, it's street driven!

check it out at http://www.bsronline.net

 

Running AC R45TS plugs, and "Xact" plug wires. (cheap advance Auto parts Brand, 7mm).

Had Autolite wires on there before (better Advance Auto Parts brand, 8mm).

 

hmmm..."cheap" plug wires. That could be your problem right there. They don't have to be cracked or anything to crossfire, basically it can it can be created like magnetic induction. Did this problem SUDDENLY appear or did you do any work under the hood?

I'm also going off the assumption that there are no vacuum leaks

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85 Berlinetta Iroc exterior NOW 454 powered, 4.10 gears, 275/60/15 Hoosier Quicktimes freshly installed th350 3200 stall, shift kit, reworked valvebody, Megashifter, Biondo linelock ..........YES, it's street driven!

check it out at http://www.bsronline.net

 

It's been doing it for so long, I can't remember if it did it suddenly.

The plug wires are *that* cheap, and the better set I just took off, had the car running the same way.

 

so it idles fine? if it idles smoothly but has problems under a load, you could have a timing advance problem. if it misses at idle, pull plug wires and try and isolate the cylinder(s). You can pick up a compression tester cheaper than a mechanic will even look at the car. go to a pep boys or autozone and I bet you can get a tester for 30-40 bucks. test those cylinders that have the problem. valvetrain is definitely a suspect on those motors. the rockers may appear to be moving just fine, sometimes they can play tricks on your eyes.

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Kyle Osterholt
Okarche, Oklahoma
ASE Master Certified
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