Backfire and Surge at Idle in "D"

S

Speed_Dreamin'

Member
Joined
Sep 30, 2005
Haven't touched my 87 GN in almost 2 years; wanting to get back to it. Last I messed with it, it left me stranded due to a crank, no-start condition. I determined there was no spark and changed the ignition module (2nd one in probably 2,000 miles or less). The engine now starts, but won't drive. It idles in park decent and has good throttle response off idle in park. When you put it in drive it starts to idle like crap and misfires and surges as soon as you press the gas pedal. Again, this was nearly 2 years ago.

I got the car started today and the above conditions are the same, though I didn't try to drive it. I let it idle in park until it was fully warmed up, put it in drive and it immediately misfired. With foot on the brake trying to bring the engine off vacuum it was surging and nearly stalling.

Here is what the scanmaster says:
O2 mV....~400 (I'm sure it's bad as I've been running leaded gas, I'll change it but I can't believe this is causing my issues?)
AF.....05
L8......35 to 40
Bat.....13.8
INT....128
BL......136
Clt......162
ATS....87
rpm.....750
TPS....0.46
IAC....10 to 22
MAL...0 (I was hoping for something here to point me in a direction)

All of the numbers seem reasonable to me except for the O2 mV and a little high on the block learn, but nothing that screams issue as bad as what I'm experiencing. Could I just have a junk brand new ignition module?

I should also point out that the car has the stock type MAF (not sure if it's original to the car or not). But the air flow number on scanmaster look fine and there are no codes so I assume it's fine.

Car and engine have ~125k miles. Stock engine w/ bolt-ons -- cold air intake, TA49 turbo, stretched intercooler, 2.5" up-pipe, 009 injectors, walboro 340 pump, adjustable fuel pressure regulator (set correctly), 3" down pipe, turbo tweak chip burned for the combo.

Any thoughts on where to start trouble shooting? Thanks guys.
 
I would borrow a known good MAF ... A Simple test. I know I have been stranded by a 'good' stock style MAF before.


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I installed a new 3.5" LS1 MAF with a new translator, 6 new plugs, new O2 sensor, ignition module is also new. Coil resistance is between 11.9 and 12.5 mega ohms, cylinder compression is between 118 and 130 psi on all cylinders, messured cold, without disabling the injectors.

I can now drive the car, but it has an occasional miss under light load, and misses significantly as soon as the turbo builds boost.

So, the new parts have made an improvement, but the issue still persists. I haven't measured resistance of the plug wires, but that would be a simple thing to do. Any suggestions on other things to look at?
 
Yes, k ohm on the coil pack. Just measured resistance on the plug wires as well. Cylinders 1&3 read ~5 mega ohms, all the others were 5-11 k ohms. Also, the plugs on cylinders 1&3 were black as could be, all others looked fine. I'm going to order some new wires. Fingers crossed.
 
(y) Let us know if the new plug wires solve the issue. Often times a miss under load will point to an ignition issue. Would be nice to closeout this tread with a resolution for the next guy that comes along with this problem :)
 
Plug wires didn't fix it. I can at least get ~5lbs of boost now. Before the plug wire change it would miss at ~0 to 1lb and sometimes less. It's now pretty smooth up to ~5lbs but then it starts breaking up really bad. Now showing code 42 (C3I EST or Bypass Circuit Failure), which I didn't have before, so I'll start on the trouble shooting tree for that.
 
Breaking up under load is often times a coil pack/module issue. A code 42 could be confirming this. Ohming your coil pack is a good quick test but is not a guarantee all is well with it. Try and find someone near your with a known good coil pack AND module and swap it out. Ideally, coil pack and module should be changed out as a unit. A bad coil pack can take out a perfectly good module. If you can't find anyone local to you with one, PM me.
 
I did a few of the checks from the trouble shooting tree, here: http://www.vortexbuicks-etc.com/code_42.htm, but haven't completed the tree. The code 42 appears to be intermittent. I checked for continuity from pin B4 at the ECM to pin A at the ignition module connector and at pin D5 at the ECM to B at the ign module, all is good. I measured the resistance at pin B4 as described in step #2, resistance was low and passed this test. I decided to replace the ignition module and coil anyway with new aftermarket parts. First start up the idle seemed smoother than ever, with no dips or stumbles, but the code popped immediately. I didn't drive the car at this time. Tonight I cleared the code from last night and started it up, idle was worse than last night with dips and stumbles, but no code. Tried to drive it, but it was worse than the last drive, basically stalling with any throttle input, then coming back up when I let off the throttle. Back to the rest of the diagnostic tree.....I guess.

The weird thing is that as I replaced parts -- MAF, plugs, wires, it was getting better. Then I replaced the ignition module and coil and it got worse, basically back to the beginning.

Any thoughts?
 
I should mention that the car has a Bailey Engineering 2-step. Could this be causing any of the issues I'm having? Also, a power logger has been on my list of things to get for the car for a long time...I don't have one, if I did, would it help to diagnose this problem?
 
Well, I most recently replaced the ECM and the ECM connectors on the wiring harness, as they were badly corroded. I was hopeful this would be the fix. It wasn't.

So, to recap, I've replaced the following:
LS1 MAF and translator (previously had original MAF)
ignition module
Ignition coil
Plugs and wires
ECM
ECM connectors
Battery

I've measured resistance from terminal to terminal in the EST and Bypass circuits, they read 0 resistance. I assume this means the circuit is good.

Still sporting code 42, still misses, surges, wants to stall under any load. Car is not drivable.

Any suggestions on where to look next ?
 
Well, I most recently replaced the ECM and the ECM connectors on the wiring harness, as they were badly corroded. I was hopeful this would be the fix. It wasn't.

So, to recap, I've replaced the following:
LS1 MAF and translator (previously had original MAF)
ignition module
Ignition coil
Plugs and wires
ECM
ECM connectors
Battery

I've measured resistance from terminal to terminal in the EST and Bypass circuits, they read 0 resistance. I assume this means the circuit is good.

Still sporting code 42, still misses, surges, wants to stall under any load. Car is not drivable.

Any suggestions on where to look next ?
 
Well, I most recently replaced the ECM and the ECM connectors on the wiring harness, as they were badly corroded. I was hopeful this would be the fix. It wasn't.

So, to recap, I've replaced the following:
LS1 MAF and translator (previously had original MAF)
ignition module
Ignition coil
Plugs and wires
ECM
ECM connectors
Battery

I've measured resistance from terminal to terminal in the EST and Bypass circuits, they read 0 resistance. I assume this means the circuit is good.

Still sporting code 42, still misses, surges, wants to stall under any load. Car is not drivable.

Any suggestions on where to look next ?
 
This is from the GN/TType.org website. Hope it helps. Code 42:
Typical causes for this code include:

1) BYPASS line is open or grounded
2) EST line is open or grounded
3) PROM or CALPACK not seated properly in the ECM
4) Poor connections between ignition module and ECM
5) Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise)
6) Faulty or incorrect ignition module
7) Faulty ECM
 
Any time my car has backfired it has been because of an air leak . Most of the time it was the rubber hose that went to the bottom of the turbo. I would make sure all hoses are tight and in good shape .
Good luck
 
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