Intermittent Stumble Cruising and not Revving Out

[1987 Grand National]

I've owned my '87 GN for about 2 months now, and it has a miss/stumble cruising under light load and will not always rev passed 4000-4200 rpm, but sometimes it will. It just stops pulling and sits there. It seems worse with engine warmed up and also in warmer weather. I think it's probably coil pack/ignition control module but I'm new to these cars and wanted to ask before I wasted my money.
Thanks Everyone

 

[turbocamino1]

I've owned my '87 GN for about 2 months now, and it has a miss/stumble cruising under light load and will not always rev passed 4000-4200 rpm, but sometimes it will. It just stops pulling and sits there. It seems worse with engine warmed up and also in warmer weather. I think it's probably coil pack/ignition control module but I'm new to these cars and wanted to ask before I wasted my money.
Thanks Everyone

Hello, Its likely that a miss/stumble from either coil pack,wires,plugs,would be accompanied by a check engine light.Could be a MAF sensor too? Have someone check for codes or if you have a scanmaster,you can check it yourself. And FWIW, if you dont have one,that would be job one as you can monitor all the settings required to have a sound running GN.

 

[1987 Grand National]

I do have a scan master but no codes.

 

[1987 Grand National]

I took the car out again tonight and I saw that the O2 sensor is about shot so hopefully that will fix it. Although it should not effect WOT correct?

 

[1987 Grand National]

Seems better after the O2 sensor, I don't feel the stumble anymore but still not revving. I'm only seeing around 4500 rpm.

 

[TurboBuRick]

Hello and welcome to the site!

Can you give a little more details about the car? All stock or?

Has it ever gone over 4500 rpm? Or did this start after you had the car a little while?

Rick

 

[SCOOBY DOO]

If the engine feels like it's hitting a brick wall at 4500 RPM's...then I'd guess valve springs.

 

[1987 Grand National]

The car is not stock. 67mm turbo, 72lb injectors, Turbo Tweak 93 octane chip, Walbro double pumper w/hotwire, and a few other things. I bought the car as it is and the previous owner did say he put comp cams valve springs in it not long ago. I believe they were done by Jim Dunn at Buick Turbo Performance.

 

[SCOOBY DOO]

If the springs are 980's...they won't take long to soften up with higher revs.

 

[phrosty]

If it skips while under acceleration than it might be a bad ignition control module.

Real easy to check with a test light. crank the engine. see if it lights up.

I had a faulty coil pack followed by a faulty ICM
Neither set off any check engine lights or any scan master codes.

When my coil pack took a shit the car would only run on 4 cylinders.
Made the car sound like it had a major exhaust leak, felt like alot of back pressure in the exhaust,
wouldn't build more then 6 psi of boost, and had zero power. Didn't throw a single code!

When my ICM shitted out I would be running WOT and the car would just cut out,
almost like a skip for a few seconds...then get back on it...then skip....repeat.
It was super annoying because it seemed like it went away, then it would just skip again out of the blue

 

[TurboBuRick]

If it pulls smooth to the same rpm evrey time then flattens out I would take a look at the valve springs like scooby says..
Do you know what springs where put in?

 

[phrosty]

If it pulls smooth to the same rpm evrey time then flattens out I would take a look at the valve springs like scooby says..

x2

Have you been experiencing any back firing?

 

[1987 Grand National]

I am also going to change the plugs just because I haven't checked them. Any recommendations on what to put in?