89 C4 with mystery misfire/stumble!

Corvince

Corvince

Casual User
You're 5
Jun 6, 2016
11
1
Manitoba
VetteCoins
501
Car
1989 Corvette Coupe
Hi. Hope someone has experience with this problem.
1989 C4 coupe with auto.
Problem: misfire at idle and under load. Intermittently clears for a few seconds then back to misfire.

Replaced:
  • New injectors (from Fuel Injection Connection, stock size)
  • " plugs
  • " wires
  • " fuel filter
  • " distributor cap, rotor and module
  • " intake manifold gasket (leaking china wall)
Tried:
  • Timing set at 6 degrees BTDC
  • Fuel pressure check (40 #) Gauge bled down to zero after 10 min. Clamped return line, slowed pressure loss, clamped both return and feed lines, still bled down slowly. Might be that I used locking pliers to clamp off lines and pressure bypassing these.
  • Checked fuel pressure at WOT during test drive and all was good. Pressure stayed at between 38 and 44 pounds.
  • Injectors firing (noid light check)
  • Cleaned MAF
  • Vacuum test with propane
  • Compression test (all cylinders good and close together readings)
After all this, there is no change in the misfire/stumble.
Any ideas for a fix? I'm baffled.
 
Nelson84 said:
Recheck your plug wires my 85 was doing that and I had 2 wires in the wrong spot. Or it could be something else. But you did change the plug wires.
Click to expand...
I will have another look at the plug wires, but the misfire was there before I did any of the things listed.
I wonder if the pickup coil in the distributor might be the cause. If it were cracked or bad in some way, could that cause a miss or would the car not run at all?
 
Nelson84 said:
I'm no expert. Just a parts changer. I just keep adding new parts until it runs right. I figure it's 30 years old probably needs new stuff anyway.
Click to expand...
I don't mind adding the parts so much, like you said the parts are 30 years old anyway. I just don't like it when the parts to be changed are a PIA to get to. I believe you have to remove and disassemble the distributor to get to the pickup coil.
 
onthe older ignitions there was two wires that ran from the pickup to the module. they would flex and break inside the insulation. caused the same symptoms. yes dist has to come out to change that pickup.not sure if same in 89 but worth a check.
 
Rmktim said:
onthe older ignitions there was two wires that ran from the pickup to the module. they would flex and break inside the insulation. caused the same symptoms. yes dist has to come out to change that pickup.not sure if same in 89 but worth a check.
Click to expand...
I will check the resistance between these two wires. I believe it should be between 500 and 1500 ohms. If I wiggle the wires with the ohm meter connected and the reading fluctuates, then one of the wires is bad and out comes the distributor.
Thanks Rmktim.
 
I found the problem!
when I was looking under the hood while the car was running, because I didn't have a light on in the garage, I saw a bright blue spark between the right front plug wire cap and the head. When I pulled the wire off I found that the porcelain part of the plug was cracked. this plug is a giant pain to get to (air con and air pump stuff in way). I could get a socket with a swivel on it, but it seams that as it was tightened the angle of the swivel was too much and the porcelain would break. I found that if you go in through the wheel well opening at the A arm you get almost direct access to this plug.
Put in new plug and the car runs great. I guess that 9 times out of 10, solutions end up being the simple things.
Thanks to all for the input.
 
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Reactions: Keith Tedford
Well done. Indeed a great find.

I have a couple of "spark plug" sockets that have rubber inserts in them. I have always felt that while it may be handy for the rubber to hold the plug while inserting it into the head they can put stress on the plug itself when working on any type of angle. Broken a few on gas powered equipment that tend to have awkward angles.
 
Last edited:
Congrats Corvince...... on an easy (and cheap) fix .... Back a couple hundred years ago in my late teens..., I dropped a 351 Cleveland in an old Ford truck on a Friday afternoon and left Nova Scotia for Cavendish, PEI two hours later.... After a weekend on the beach, we started back towards home Sunday night and all of a sudden there was a loud bang and then a rhythmic pop,pop,pop,pop,pop.... Got out fearing the worst...Opened the hood and also spotted that telltale blue sparking.... cept mine wasn't anywhere near the heads... lol... Some idiot (me), in his rush to get to the beach, didn't tighten the spark plugs and one had worked it's way out and compression blew it the rest of the way out....lol.... Luckily, it was still attached to the plug wire and I was able to finger tighten it back in (who carried wrenches to the beach?), and it got me the last couple hours home.... Gotta love a happy ending.....:Cheers2:
 
in the future with an unexplainable misfire add this test to your checklist: when it is dark outside, or in your garage just for a second with the car running get a spray bottle of water run the car and spray water down along the side of the heads where the plug wires are and look for a light show of Sparks. Whenever you think you have an ignition problem make sure that you include that in your test.
 
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