I'm begining to trouble shoot this bad boy tommorow, but I'll go ahead and get this out there.

I have a 91 vette, TPI. I have already replaced the coil, cap, rotor, and wires on this car 2 days ago.

I took my test light, and set the timing with the EST bypassed. Then hooked the est bypass wire back up.

I can rev the car up, and snap the throttle shut, and it will backfire through the exhaust several times. If I put this car under a load, ie...40 mph in overdrive, around 40% or less Throttle opening, it will skip and sputter. It skips and sputters all the time, but cleans up when the RPMS climb. Seems the higher the better.

I took my test light, hooked the lead on the wires, and noticed that when I hear the backfires, a couple of cylinders are not recieving spark from the distributor. I noticed # 8 and 7 was the worst cylinders, but there were a few more cylinders that would misfire ever now and then.

Can the module or pick-up coil have anything to do with this? I figured if the pick-up coil was the problem, then it would do this on all cylinders. Module, I'm not sure about this part.

What do yall think?

I'll begin my troubleshooting tommorow.

D Moss

 

What year ,what mods ,Compression on all 8 cylenders.

 

sure you have the wires going to the right pistons?

 

I had this same problem earlier this summer. I also noticed an irregular spark when using the timing light.

The module in the distributor was bad.

It's an easy enough task to remove the module and have it tested. Autozone has a machine to test it. I forget exactly the cost but it was less than $20.00. Don't forget to use ALL the grease on the underside when you replace it!

 

Old plugs maybe?

 

Pull the vacuum hose off of the EGR valve and plug it. Then go for another ride. Any change?

RACE ON!!!

 

I just replaced the module, it wasn't but $18, so I got it.

Still misfiring on cylinders.

I'll be replacing the plugs tommorow, and I'll check the EGR valve, and take the vacume off of it to see what happens.


Wires are going to the right cyclinders, it is misfiring, as a "no spark" condition. The backfiring in the exhaust is comming from unburned air/fuel mixture from the non sparking cylinders.

Everything on this car is stock also.

Still searching.

D Moss

 

Dirty throttle body can cause that hesitation too. Pull it off and clean it up real good, check the IAC and TPS, and also the MAP sensor.

 

Still sounds to me like a few bad plugs...has the car been sitting for a little while...what's the mileage?

 

Have you tested your injectors?

 

It appears that you have isolated the fault to disappearing spark when the misfire occurs. For this reason, I think you can forget about the EGR for now.

I'd suggest you carefully and thoroughly check all of the connections and wiring on the low-voltage side of the ignition system.

I had a similar problem years ago on an Olds 403 engine with HEI. In troubleshooting the fault, I discovered that a wire connecting the ignition module, inside of the distributor, had fatigued with age. This wire was broken inside of its insulation, such that the break was not visible from the outside, but would cause an intermittent electrical connection to the module. When the vacuum advance rotated the breaker plate, it disturbed this wire, and the connection would get erratic.

Look for an intermittent connection anywhere on the low-voltage side of the system.

Be well,

SJW

 

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Is there a pick-up in the distrib?You may have got a bad cap or rotor,or try taking coil and cap,rotor off and putting it back on.

 

You Either Have The Plug Wires Crossed Or You Got Bad Spark Plugs, So I Would Just Replace The Plugs And Check The Plug Wires Again And Make Sure You Don't Get Them Crossed, The Exact Same Thing Happened To Me.

 

I've replace all the plugs, wires, cap, rotor, module, and coil. Everything is new!!!

I checked the compression on the cylinders, all came back around 200 to 210 psi.

I checked the fuel pressure, without vacume, at it came in at 42psi. With vacume it was around 37psi. FPR was working. Injectors didn't seem to be leaking through either, fuel pressure was solid at 42, did not drop.

I check the EGR valve with a minivac system, put a vacume on the diaphram, and my vacume on the gauge went away! I'm thinking that the diaphram on the EGR valve is broken. I plugged this vacume line for now.

I got a code on the EGR system 2 days ago. I'm thinking about taking the EGR off totaly. The valve seat may not be sealing.

I checked my vacume on the engine, and my vacume was reading a solid 20psi! Open the throttle, vacume went away, snapped the throttle shut, and a 25psi vacume peak, slowly comming back down to a steady needle of 20psi.

I took my timing light and doublechecked my timing at 6*btdc, which is what my tag on the radiator said to set it. It is right.

I saw no real mechanical evidence on what is happening, so I'm gonna recheck my tune in the ECM and see what is going on. I'm copying with datamaster now.

It's only when I put the car under a load in gear, it begins to shake, vibrate, and pull timing like crazy. Timing in the ECM isn't as high as stock, around 36 or less, and datamaster is showing the same thing. Unloaded, the engine will rev quick, when I snap the throttle shut, it will pop pop pop through the exhaust.

Still looking, and thanks for the support. I'll keep yall posted.

 

Double check, then triple check the wires. I had a backfiring issue and I would have bet a bundle of cash that I had the wires correct. I traced, traced and traced again and still had the issue. I called a friend and explained the problem I was having. He said to trace the wires, I told him I did, a dozen times, trace them again he replied.

I did and #1 and #3 were crossed. It doesn't take much to create an optical illusion, you look at a wire and think it's going to cyclinder #1 when in fact it's going to #3. Grab each wire and follow it to the correct cyclinder.

Good luck.

 

Don't hold me to this, but I thought if the ignition module was bad the car wouldn't start at all. Two years ago I had an intermittent starting problem. I troubleshoot the fuel pump, pickup coil, changed the fuel filter just to name a few. Turned out the ignition module was "breaking down" causing the intermittent starting issue. I replaced the ignition module and the car has been fine since.

 

Believe it or not,.......

It was a bad injector. One injector on the passenger side was bad. The o2 sensor is on the driver side, so it wasn't picking up the raw o2 comming in the exhaust stream.

Backfiring was from the o2 being thrown into the exhaust.

Replaced the injector.

Car is running great now, and thanks for all the ideas and help troubleshooting this problem.

D Moss