HELP! Engine Issue, can't diagnose!

[93formulalt1]

I cant confirm or deny delphi quality, but iirc delphis have a slightly different color to them, so i doubt they are a "rebranded" AC Delco wire.

I'll get the Delco's then. They are a special order part, so the car is going to limp home, sit in the garage, and when I get around to swapping the plug wires, I'll do it. Was really hoping the car would be running better by now, but if it's only going to cost me another $120 to rule out the ignition system, I'm in good shape.

I think I'm going to pick up the true duals listed here (hopefully I can make it happen tomorrow, before I head back across the state), swap my headers (since I can't access the plugs with the shorties in place, and it's MUCH easier with LT's), replace plugs and wires again, and if that straightens out my issues, I'll start focusing on other issues, including the clutch, suspension, windshield, t-top seals, stereo wiring, etc.


This car needs a lot of work, don't get me wrong, it's no cream puff. Clean body, miniscule amounts of rust, but it's still a project, that's for sure. But then again, when are our cars ever finished?

[Darren]

If you didnt replace 6&8 i would bet thats your issue right there. Replacing 6 of 8 plugs is doing nothing good for your car.


Sent from my iPhone 5 using Tapatalk

[93formulalt1]

If you didnt replace 6&8 i would bet thats your issue right there. Replacing 6 of 8 plugs is doing nothing good for your car.


Sent from my iPhone 5 using Tapatalk

I agree, it wasn't laziness, it was simply a lack of time and facilities to do the job right, and I intend to correct the problem (as mentioned above, swapping headers and replacing plugs and wires). Only reason I didn't go back the following weekend and do those two plugs was because the other 6 plugs looked fine, I just replaced them because they were easy enough, and cheap. I'd bet they are all fouled by now, as the last time I pulled #1 & #2, they smelled like gas. . .

[meissen]

This is exactly what I told Aaron when he came to me for advice on Larry's old car, and the car hasn't had a single backfire in 3 weeks. Problem is 100% solved.

Don't be so sure. Dad's car would run good when it was cool out but the backfiring was worse when it was 80*+ outside and the car sat in the sun for a few hours. I'm hopeful the one wire fixed it, but I'm doubtful.

[JoeliusZ28]

He's been driving it quite a bit to work and hasnt had any issues with it yet

[farmington]

start it up and run it up to temp in the dark and look for shorting plug wires. If the wires are shorting to ground you will see it.

[mazakman]

Do the old spit on the finger test on the headers. Tells you which cylinders are not burning hot. Great way to find a fouled plug or bad wire

[93formulalt1]

Do the old spit on the finger test on the headers. Tells you which cylinders are not burning hot. Great way to find a fouled plug or bad wire

Or I could use my infrared thermometer. I'll find it and do that this afternoon.

[TrickyTransAm]

The other way may be to hook up an OBDI scanner and check for Misfires, and which cylinders are causing the havoc.

I ordered a half height spark plug socket off ebay. It's use works on B cars as well as F-cars with the LT1. Stupid expensive, but necessary for these types of cars.
FWIW, I put Taylor Pro ignition wires on my co-workers B-car LT1 about 70K miles ago and he's been happy since. No issues on his daily winter driven "Beater" as he calls it.
Delco/Delphi are pretty much interchageable. Delphi has been making stuff for GM for years.
I would not use Autolite or Durolast wire sets. I've had multiple sets rot off in record time. They just aren't worth the money.

IMO backfiring isn't normal. Definitely do as the others have said - get Plug 6 and 8 swapped, re-check your wires, re-coat with dielectric grease and re-install.
If it's still backfiring, I'd almost think something might be going on with Timing - mabee a stretched timing chain is throwing things off.

If it's that much of a pain, It may be time to take it into a garage and have it fixed. If it's a daily driver the tow bill will far outweigh the repair cost.
At a minimum you'll at least know whats wrong.

[93formulalt1]

The other way may be to hook up an OBDI scanner and check for Misfires, and which cylinders are causing the havoc.

I ordered a half height spark plug socket off ebay. It's use works on B cars as well as F-cars with the LT1. Stupid expensive, but necessary for these types of cars.
FWIW, I put Taylor Pro ignition wires on my co-workers B-car LT1 about 70K miles ago and he's been happy since. No issues on his daily winter driven "Beater" as he calls it.
Delco/Delphi are pretty much interchageable. Delphi has been making stuff for GM for years.
I would not use Autolite or Durolast wire sets. I've had multiple sets rot off in record time. They just aren't worth the money.

IMO backfiring isn't normal. Definitely do as the others have said - get Plug 6 and 8 swapped, re-check your wires, re-coat with dielectric grease and re-install.
If it's still backfiring, I'd almost think something might be going on with Timing - mabee a stretched timing chain is throwing things off.

If it's that much of a pain, It may be time to take it into a garage and have it fixed. If it's a daily driver the tow bill will far outweigh the repair cost.
At a minimum you'll at least know whats wrong.

Problem is, I've already had 2 mechanics look at it. Both think its a timing issue. If it were timing, it would have started all at once, not a minor misfire, slowly progressing to a full-on backfire. I'll swap the wires and plugs, and if that doesn't fix it, I'll pull the timing cover to check the timing chain.

[hoogiesngrinderz]

What concerns me is the 10 inHg while at idle. My big cam 355 idles 10-12 inHg.

Terrible gas mileage, backfires, odd MAP readings...I'd be looking for decent size vacuum leak.

[93formulalt1]

What concerns me is the 10 inHg while at idle. My big cam 355 idles 10-12 inHg.

Terrible gas mileage, backfires, odd MAP readings...I'd be looking for decent size vacuum leak.

Sprayed carb cleaner all over the place yesterday, no indications of a vacuum leak. How much vacuum should the engine produce at a 750RPM idle?

[hoogiesngrinderz]

Stock cam engines normally idle around 18-20 inHg.

[WMCC Gary]

If all else fails...

Place these just behind the right front tire, Light and Run like hell.



You should get a little enjoyment out of knowing you may not have fixed the problem but you got the last laugh.

[hoogiesngrinderz]

Check the PCV valve, make sure it is clean and not stuck open.

Pull and plug the brake booster vacuum line to rule out a bad brake booster.

Check to make sure EVAP purge solenoid isn't stuck open.

There is a vacuum elbow that commonly cracks on LT1's, can't remember what one off the top of my head.


Just some things I'd check first. They're free