New engine, has a misfire ('82 Turbo GN)

b4black

Well-Known Member
Joined
May 24, 2001
This spring, I installed a new engine in my '82 GN. Great news is it starts, runs, etc. Bad news is it has a misfire. The engine was a used short block, to which ported heads and a roller cam were installed.

At idle, it feels fine. In gear, it stumbles and misses. Feels pretty consistent, like a couple of cylinders aren't firing. Distributor, cap/rotor, wires and plugs are new. I've quadruple checked the wire routing and it's correct. (1-6-5-4-3-2). I also put a spark tester in line and each cylinder is firing (at idle and in gear).

I did a compression test. All cylinders were between 130-136 psi.

Suggestions on next step?
 
You can use a common 12 volt test light to short out cylinders at the plug boots to find out which cylinders are dead. You may have a vacuum leak also. You can use some carb cleaner to spray around possible leaks. I use a propane bottle with an adjustable valve and hose to check for lean conditions such as vacuum leaks.
 
Thanks for the suggestions.

I tried a another new set of plugs, and rechecked all the vacuum lines (found a very small leak at the cruise control and capped that off). Didn't help.

I did make some progress. The #1 cylinder exhaust tube was way colder than the rest. I pulled the plug and it was wet. So I now know #1 is the misfire (I'll take this over a random misfire).

I swapped the plug wire for another, but no improvement. Next will be the (new) distributor cap.
 
Definitely sounds ignition related. How do the other plugs look compared to that one?
 
If the cap doesn't solve your problem, I would pull the valve cover on the drivers side and check the valve action. An intake valve not opening will cause a big miss. An exhaust valve not opening causes a big pop through the intake.
 
If the cap doesn't solve your problem, I would pull the valve cover on the drivers side and check the valve action. An intake valve not opening will cause a big miss. An exhaust valve not opening causes a big pop through the intake.
 
I swapped the cap/rotor/coil and that didn't fix the problem. And just to be 1000% sure it's not the distributor in any way, I rotated it 60° and move each wire back one position (so if the problem where somehow in the distributor, the miss would move to #2.). That didn't help either, so I'm confident it's not ignition related.

If the cap doesn't solve your problem, I would pull the valve cover on the drivers side and check the valve action. An intake valve not opening will cause a big miss. An exhaust valve not opening causes a big pop through the intake.

I was hoping the compression test would rule out a valve issue. But I see what you are saying. The intake must be opening a little because it builds compression and the plug gets wet. They must be closing completely because it build compression.

I've been worried it might be something like this all long. When I bought the engine, the seller made some odd comment about the heads coming from an engine that had the wrong push rods installed. I also suspect he may have used a mismatched set of used rockers and/or pushrods. The cam and lifters are new, so hopefully they aren't the problem.

Ugh, time to pull the turbo/carb assembly and look under the covers.
 
Update:
- I found out you can pull the carb plenum off without removing the turbo. The bottom bolt it tricky, but possible.
- I popped the valve cover off and all the valves look good. Up and down - I measured the lift and they all fully open.
- I checked compression again. #1 was 135 psi and #5 was 133.
- Next pulled the turbo and stuck a borescope down the intake. Didn't find any 10 mm sockets or any other obstructions.
- I believe I did find the problem, but I want to get it back together to confirm before I declare sucess.



4.1 dsvc.jpg
 
Fixed!

It was a vacuum leak (as suggested above). If you look at the picture above, you'll see a mounting boss and bolt hole cast onto the number #1 intake port. The hole goes all the way through to the port. D'oh! It doesn't act like a typical leak, since it only effects one cylinder.

The bolt would hold a plenum bracket, but it unused on the 82/83 engine. It's been staring at me all along, but it wasn't until I saw the hole from the inside with the boroscope that I figured out is was open.

Now if it stops raining, I can go for a drive. :)
 
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