Hesitation / missing when hot.

[Chris Gullett]

I am hoping someone on here can help me with a problem with my 87 GN. The car starts fine, idles fine and drives fine for about 45mins or until it is fully warmed up. Then it starts to have a hesitation or starts missing. Most of the time this happens around 2000 rpm, sometimes right from idle. At 60 mph you can feel a steady miss. The blms at 60 mph are low, about 105. Then the miss/hesitation will go away for a few minutes.

The car is bone stock, 20k miles. It has been in storage for the last 20 years. So far I have done the following:

• Gas tank flushed and new fuel pump installed
• New fuel filter
• Oil & filter
• Air filter
• Replaced coil and control module (Delco stuff from nos4gn)
• spark plugs and wires

Scan master readings at idle are:

O2 = moves from 0 to 700 (approx)
AF = 5
LB = 39
Bat = 13.5
INT = 130
BL = 122
TPS = .36
IAC = 19
CC = moving, but on occasion moving slowly

I tested the old coil and it was open on 2 of the packs, the 3rd was at 12,000. The new one tested 11,000 on all three. I thought this or the ICM was the issue. It seems I was wrong. The MAF passes a tap test. Any help would be appreciated.

 

[josh young]

I am hoping someone on here can help me with a problem with my 87 GN. The car starts fine, idles fine and drives fine for about 45mins or until it is fully warmed up. Then it starts to have a hesitation or starts missing. Most of the time this happens around 2000 rpm, sometimes right from idle. At 60 mph you can feel a steady miss. The blms at 60 mph are low, about 105. Then the miss/hesitation will go away for a few minutes.

The car is bone stock, 20k miles. It has been in storage for the last 20 years. So far I have done the following:

• Gas tank flushed and new fuel pump installed
• New fuel filter
• Oil & filter
• Air filter
• Replaced coil and control module (Delco stuff from nos4gn)
• spark plugs and wires

Scan master readings at idle are:

O2 = moves from 0 to 700 (approx)
AF = 5
LB = 39
Bat = 13.5
INT = 130
BL = 122
TPS = .36
IAC = 19
CC = moving, but on occasion moving slowly

I tested the old coil and it was open on 2 of the packs, the 3rd was at 12,000. The new one tested 11,000 on all three. I thought this or the ICM was the issue. It seems I was wrong. The MAF passes a tap test. Any help would be appreciated.

It might be worth putting an 02 sensor on it, especially if it's the original one.

 

[Chris Gullett]

Thanks Josh, I put in a new O2 sensor this afternoon... more of the same. Doing some more reading on here, I am not sure if I am describing the problem correctly. It is like the engine is very briefly cutting out, bucking. Not all the time. Only when warm and then maybe about 60% of the time. Usually at about 2000 rpm, but sometimes stumbling upon initial acceleration.

 

[josh young]

I had the same issue with mine but it was at about 15 pounds when it started. It was fine at: normal driving, and moderate throttle when hot and even full throttle when cold. The issue arose when you got it hot and stepped on it all the way. It run clean with no knock when this was going on. Everyone said most likely source was either a coil pack or a module. I had a new coil pack put up so I stuck it on. when I took the old one off I discovered the module had to texture of melted laffy taffy. I thought sure I had found my problem, put new module and coil pack on, more of the same. People figured I had got a bad one of the two. I just couldn't get past it acting the exact same was as it did before. I was reading one night on a thread I started, it's still on here I guess, and one member told me to check the cam sensor cap. The more I read on it the more I thought it was worth a shot. I ordered a new one and bingo it was just like it was before the trouble. It would be cool if you knew another turbo regal guy close by that would let you swap to verify. That may not be your problem but in ways it sounds similar to mine. How does it run if you step on it? Do you show any knock?

 

[Chris Gullett]

Since getting it running again, I have opened it up fully once and the KR climbed to 8 degrees and I let off... I figured I would get the drivability sorted first then the WOT issues.. but I guess they are related.

 

[josh young]

You got any way to see if fuel pressure is going up correctly?

 

[Chris Gullett]

Well, following your advice. I took it out today and drove the car until it started acting up, it was a steady bucking at 1700 rpm. I pulled over and unplugged the cam sensor and kept driving. The stumble was gone, drove for another 40mins and it didn't act up at all... so is the sensor bad? Is this just as simple as replacing the cam sensor cap?

 

[kandc7]

Your tps is low. It should be around .42

 

[josh young]

Hey Chris. You had a problem you eliminated X from the equation, problem gone. I guess it could be coincidence but I'd bet you found your problem. You have 3 options: 1.) Buy the whole shebang to get just the cap you need, the parts stores don't sell just the cap. 2.) Buy a Casper's cap with led light - I didn't read much good on these. or 3.) Buy one from the company that made them in the first place - Tomco. The unit from Tomco was about $50, you'll probably think it's some of the best money you spent. If you need a part # I will dig around and find it for you. Also check the phillips head screw under the cap to make sure it's tight while you have it off. Good luck Chris!

 

[Chris Gullett]

I did a bit more reading and tried turning the cam sensor approx 2 degrees CCW, I also pulled the cap off and made sure the reluctor was tight, which it was ... The car now runs like a top, all of the bucking and hesitation is gone. It must have been right the very edge of being correctly set, then once warmed up, the heat from the engine maybe gave just enough expansion to push it over the edge and run poorly. Just a theory... thanks Josh for getting me on right track!

 

[josh young]

Glad you got it running right Chris!