1987 Buick Grand National - 231 v6 - random misfire at idle when warm - no codes

1987 GN that has had a persistent random idle miss when warm. Does not appear to have it when cold or off idle. Engine speeds up and down randomly 800 - 875 rpm and coincides with the miss. Also have an 1989 Turbo Trans Am that has the exact same engine that runs %100 perfect. Have swapped every single bolt on part, except the crank sensor, from the TTA one at a time with no improvement in the GN or degradation in the TTA. The hydraulic cam was beginning to wear out so I hired a shop that specializes in these cars to replace the cam with a roller cam and put in aluminum ported heads as well, hoping it was a valve train issue. Cam is a Comp Cam 206 206 which is mostly stock except for a bump in the upper mid range. Car has more power but still has the exact same miss issue. Confident there is no change in idle quality with the cam and head swap - no worse no better. Replaced fuel injectors as well with new ones with the correct matching chip. Fuel pressure looks to be stable. Bought a smoke tester - no vacuum leaks. Ran a shop vac in blow mode into the tailpipe and verified no pre O2 exhaust leaks. EGR has been permanently blocked.

With the car running if I unplug the cam sensor the car runs rougher and obviously throws a code. When you unplug the cam sensor the car is supposed to throw a code but run in batch fire mode and have no visible change in idle.

Did the screwdriver check for the injectors - never heard one drop out.

Chip is designed to run in open loop at idle. At idle INT stays on 128 and BLM for the most part stays on 121. IAC is between 13 -15. IAC at pure idle should not wander. So the car is running rich and the computer is pulling fuel out. Wife has been complaining when I go back in the house after working on it because I smell like gas. I was reading that you can tell something about what the engine is doing by the smell of the exhaust but can't remember what it was? Maybe if it is burned or unburned gas?

Have an aftermarket ignition system designed just for these cars that has built in diagnostics to tell you if you are having crank or cam sensor issues. www.tr6.bailey-eng.com/TR6%20User%20Manual%201.1.pdf Never throws a code other than when I unplug the cam sensor.

Attached pics of one spark plug. I took out all 6 and they all look the same. Rich.

The only parts I have not changed on this car are the crank sensor and fuel pump. Crank sensor is original and fuel pump was changed in 98'. I have both and will likely change them this weekend.

How rough does it run?? Also the chip you have runs in open loop?? You swapped every piece from car to car. Maybe try swapping computers? Got anymore scan data?

does it still have the CAT on it? I had a TTA and had to put the Cat back on it due to the smell and it was running fine.

Also if you don’t have scope when it starts misfiring try cooling off the crank sensor with water see if miss goes away??

I added a turbo buick specific scan tool called Power Logger which adds much more information including time lapse logging. But the Power Logger application needs to be installed on a computer. www.turbotweakstore.com/ProductDetails.asp?ProductCode=1018 If anyone wants to install the app I have log files.

Yes, the chip runs in open loop by design. The idle is not horrible I am just weird like that I guess. It used to idle smooth as glass with an old technology chip - so I know there is something wrong with it. It went from running great to what it currently is with the old injectors and old chip. The Turbo Tweak chips are known to be among the best for these cars. And now that I know it is running this rich that has to be fixed. Has to be more power in the car if the A/F ratio was right. "Lean is mean!"

I did swap the computers and have a few spares as well. Never a change.

Cat was gone long ago. 3" stainless from the turbo to the tail pipes. TTA is also all 3" stainless.

That a good idea with the water on the crank sensor. Definitely try that tomorrow.

Want to get a scope. Looking at them now.

Making progress? Misses are less and the car just sounds stronger at idle.

Replaced the plugs. Found out that it can help to reduce the air gap on the crank sensor on the outside - so bigger gap on the inside.

Tried spraying cold water on the crank sensor. Seemed like it may have helped. So I will go ahead and replace it. It is 32 years old.

Found out the Turbo Tweak chip can be set to idle in closed loop. Did that. Having these cars idle in closed loop typically makes the idle worse. So now after these changes I will try putting the idle back in open loop and see what happens.

Noticed that if I rev the engine around 2000 rpms and hold it there initially the fuel pressure jumps up about 3 or 4 pounds, as it should, but then slowly goes back down. That tells me the pump is either weak or the filter is clogged.

Misses are definitely less but I can still hear the engine speeding up and down randomly. Almost like a cylinder is dropping out randomly. I think this is where the scope would help.

Was going to get the OWON VDS1022I. What tests would I need to run to isolate this problem? Do I need any other attachments for the scope?

The random miss is %99 gone. It was the battery disconnect I have on the negative terminal. Must have had some intermittent resistance which I guess was enough to mess up something in the computer system. I did swap out the 20 year old fuel pump but that did not really change how it ran. I have 4 other vehicles with the same battery disconnect and never had an issue in 20 years.

Vacuum is up to around 18 at idle and engine sounds strong when you blip the throttle.

However, there is still some type of electrical issue. The volts will randomly drop from around 13.5v - 13.2v to around 12v at idle for a split second then bounce back. This happens maybe once a minute. These reading are from my scan. I thought the bad idle/miss was causing the voltage drop but it is not. When this happens lights dim, fuel pressure drops, and idle slows down, and the alternator sounds like it is working hard. If I disconnect the alternator exciter wire (brown 14g wire that connects on top) then obviously the normal volts are lower and when it dips It goes from mid 12v down to low 11v. Appears to be a bigger drop when the alternator is unplugged. Seems to get worse once the car is warmed up. It is not the cooling fan - although the lights do dim when it turns on.

If an alternator is connected but not "plugged in" could it still be causing this issue? Rectifier diode(s) intermittently shorting? This issue to me seems like it might be the alternator. I may swap alternators with the TTA next and see what happens.

Other than that I might just start unplugging relays and pulling fuses until it stops.