Rough engine / stalling after valve adjust

[Sdgg91]

Post is lengthy but really disappointed my car does not run so I am trying to give the full story below in hopes to get some advice.

I had a valve adjustment done as well as new tarrett engineering valve covers and valve cover gaskets put on my 3.8L (964 block) motor by a very reputable air cooled shop (not naming any names but they only work on air cooled 911s and the owner/mechanic has been in business since 1974. (He still does all the mechanic work on all the cars).

I picked up my car and after firing it up immediately noticed it was running rough. I almost attempted to pull away but instead pulled back into the shop. The owner of the shop who did my valve adjustment was there helping me back out and we pulled the car back inside since it was obvious I wasn’t going to make it down the street let alone home.

He thought at first it was maybe a low fuel or fuel pressure issue which it was not. We added gas and he later ruled out after we pulled it back inside that fuel pressure was all good.

About 600 miles ago I replaced the distributors and caps as well as plug wires and the car was running like a top before the valve adjustment. (The plug wires were arcing and it was running rough before replacing those items) but everything was running tip top.

So now he is going to try to get new coils since he thinks that is the issue but I have got to think he made a mistake while performing the valve job and reconnecting the plug wires since the car was running perfect before.

As you can imagine I’m slightly stressed and bummed that my perfectly running car went to the shop for a routine job and now it barely runs.

Is there anything else we can check looking for any and all suggestions? I did notice that top plugs 1 and 6 felt sort of loose and you could jiggle them a bit but he repeatedly said that there can be some movement and assured me the plugs wires are securely on the plugs.

We checked plug numbers to cylinders and everything matched since that was my first thought.


More about the engine build
Based on a 964 case
TWIN plugAll new Clevite coated bearingsNew steel exhaust valves1mm bigger intake valvesMulti angle race valve jobNew Valve guides for exhaustNew gasket set with machined head sealsNew timing chainsComplete new hardware set for caseRe hone barrelsCustom JE pistons 10:1 compressionNew low tension piston ring setNew drain back tubesNew Pauter connecting rodsRebuilt and blueprinted oil pumpMild porting of headsCustom Stroker 3.8 crankshaftStree/race custom camshafts

[Alan L]

One thing to do is check the temps on the exhausts - as it runs. That will tell you how many bad cylinders you have, and where they are. Somewhere to start.
Alan

[brighton911]

Duplicate

[brighton911]

"I have got to think he made a mistake while performing the valve job and reconnecting the plug wires since the car was running perfect before."

I totally agree with you. The shop must perform a proper diagnosis, not just throw expensive parts at it hoping it will cure the problem. If it were my shop, I would be most interested in making you happy. They need to make it right in the right way - the first step is to diagnose the problem methodically. Insist that they do, the car was running fine before they worked on it. Good luck

[ant7]

If the car was running fine before the valve adjust job, then its either the valves have been adjusted too tight, or something has been put back wrong, ie; plug wires etc.
Ant.

Originally Posted by brighton911

"I have got to think he made a mistake while performing the valve job and reconnecting the plug wires since the car was running perfect before."

I totally agree with you. The shop must perform a proper diagnosis, not just throw expensive parts at it hoping it will cure the problem. If it were my shop, I would be most interested in making you happy. They need to make it right in the right way - the first step is to diagnose the problem methodically. Insist that they do, the car was running fine before they worked on it. Good luck

[john walker's workshop]

He actually let the car leave the shop like that?

[Sdgg91]

Originally Posted by john walker's workshop

He actually let the car leave the shop like that?

Yeah… I’m giving him the benefit of the doubt right now to try to get it back to how it was given his reputation. it would be a big coincidence that all of a sudden the car does not run after it being there for work when it was running 100% prior to the service. It does have the original coils so I am fine replacing those but if this doesn’t work I will push for him to go through everything again.

[john walker's workshop]

Coils rarely go bad. Have him redo the valve adjustment, check all the plugs and make sure the plug wires are in order. I'e do that if it happened on my shift.

[Locker537]

I'd start with checking firing order and wires...again. Then plugs. Maybe a terminal fell off or something simple.

[boyt911sc]

Originally Posted by john walker's workshop

He actually let the car leave the shop like that?

Why did you take the car out from the shop if you knew there was something very wrong with it. It was running well when you brought the car to him and now running terrible! A responsible mechanic/shop would NOT allow a customer to drive home with the car in this condition. How long have you been dealing with this shop?

Tony

[Sdgg91]

Originally Posted by boyt911sc

Why did you take the car out from the shop if you knew there was something very wrong with it. It was running well when you brought the car to him and now running terrible! A responsible mechanic/shop would NOT allow a customer to drive home with the car in this condition. How long have you been dealing with this shop?

Tony

It’s a small shop, I moved it about 15 feet I thought it was just a rough idle that needed to warm up. But could tell things were a little off. The mechanic was there the entire time watching.

[al lkosmal]

As others have recommended...I'd start with the plug wire routing.

al

[scarceller]

I agree with others here, most likely ignition related. Maybe swapped wires?
Try running it only on the top plugs by disconnecting one coil. The do the same and run only on the lower plugs. Just to see if it runs better on just top or bottom plugs.

[Sdgg91]

Well to my surprise the coils are bad. the car still has the original coils but when replacing with the two new ones it runs normal again. He also replaced the ignition pre wires( I think that’s what he called it but maybe it’s a different name) so now the entire system is new other then rebuilding the distributor and replacing the dist belt.

I guess I was just shocked at the coincidence unless there is something that the shop did that could have cooked both coils?

[Sdgg91]

Originally Posted by scarceller

I agree with others here, most likely ignition related. Maybe swapped wires?
Try running it only on the top plugs by disconnecting one coil. The do the same and run only on the lower plugs. Just to see if it runs better on just top or bottom plugs.

Well to my surprise the coils are bad. the car still has the original coils but when replacing with the two new ones it runs normal again. He also replaced the ignition pre wires( I think that’s what he called it but maybe it’s a different name) so now the entire system is new other then rebuilding the distributor and replacing the dist belt.

I guess I was just shocked at the coincidence unless there is something that the shop did that could have cooked both coils?

[tobluforu]

WTH are ignition pre-wires, is that something on a 964? Did he give you the old parts, cause if he did, I would swap out the coils to see if the coils were indeed going bad. Or he f-ed up, had the firing order messed up and then had to come up with a reason to make himself not look like a goofball. Sorry, I'm a pessimist at times is all.
Enjoy your car.
Peace out.

[ant7]

I think there are probably more than a few here thinking similar of course he could have also swapped the working coils, and fitted bad ones, who knows, but if the OP is happy with the outcome, then I guess we should accept that and move on.
Ant.

Originally Posted by tobluforu

WTH are ignition pre-wires, is that something on a 964? Did he give you the old parts, cause if he did, I would swap out the coils to see if the coils were indeed going bad. Or he f-ed up, had the firing order messed up and then had to come up with a reason to make himself not look like a goofball. Sorry, I'm a pessimist at times is all.
Enjoy your car.
Peace out.

[scarceller]

Were the coils the 964 black OEM coils that went bad? Those coils are very rugged and they rarely go bad, to have 2 of them go bad is unheard of.

What are ignition pre wires? Are these the wires that go from the coil to the center of the dizzy cap?

A coil can get cooked if it's over charged, like keeping constant 12vdc on the coil for very long periods of time. If you are using some after market ignition system I'd suspect possible issue in that setup? It's possible that when engine is OFF the coils still have power on them? Those OEM coils have about a 4millisecond charge time at 12vdc, means they only need to be on for 4ms at most to be fully charged. See this video I created a while ago to understand charge times on the 964 coils.
https://youtu.be/a7dFdIdsZeU
The coils will get super hot if overcharged, you should be able to hold and touch the coils case and not get burned with engine running. The can temp should remain below 140F at all times. If the coil can is above 180F it's likely being overcharged.
I also gave you the max dwell time of about 4ms, I'd double check the ignition system to see what it has max dwell set at?

Originally Posted by Sdgg91

Well to my surprise the coils are bad. the car still has the original coils but when replacing with the two new ones it runs normal again. He also replaced the ignition pre wires( I think that’s what he called it but maybe it’s a different name) so now the entire system is new other then rebuilding the distributor and replacing the dist belt.

I guess I was just shocked at the coincidence unless there is something that the shop did that could have cooked both coils?

[Sdgg91]

Originally Posted by scarceller

Were the coils the 964 black OEM coils that went bad? Those coils are very rugged and they rarely go bad, to have 2 of them go bad is unheard of.

What are ignition pre wires? Are these the wires that go from the coil to the center of the dizzy cap?

A coil can get cooked if it's over charged, like keeping constant 12vdc on the coil for very long periods of time. If you are using some after market ignition system I'd suspect possible issue in that setup? It's possible that when engine is OFF the coils still have power on them? Those OEM coils have about a 4millisecond charge time at 12vdc, means they only need to be on for 4ms at most to be fully charged. See this video I created a while ago to understand charge times on the 964 coils.
https://youtu.be/a7dFdIdsZeU
The coils will get super hot if overcharged, you should be able to hold and touch the coils case and not get burned with engine running. The can temp should remain below 140F at all times. If the coil can is above 180F it's likely being overcharged.
I also gave you the max dwell time of about 4ms, I'd double check the ignition system to see what it has max dwell set at?

Thank you that is very helpful and I appreciate all the comments! I’ll do some testing on the old coils.

I can’t say I’m 100% satisfied since the whole situation is a bit odd. I also would have liked to do the 993 coil conversion since now I have 2 silver coils instead of the original 2 black Bosch ones. I have heard good things on that retrofit.

Ignition system is stock motronic nothing aftermarket other then Steve Wong tune.

[scarceller]

But you MUST have some sort of solution that fires 2 coils that sits between the stock DME and the 2 coils. You can not fire 2 coils directly from the stock DME. Years ago folks used the Andial Splitter solution and now I also offer a correct better solution for this also. It's very important that you understand how those coils are being controlled, it can't be done directly from the DME. You can not and should not attempt to fire 2 coils at same time from the DME directly.

The silver coils are not as good but you can always find good working black 964 coils on ebay all the time. The 964 coils are more robust than the 993 coils, I'd suggest a set of used original black 964 coils over the silver ones and also over the 993 smaller coils.

Originally Posted by Sdgg91

Thank you that is very helpful and I appreciate all the comments! I’ll do some testing on the old coils.

I can’t say I’m 100% satisfied since the whole situation is a bit odd. I also would have liked to do the 993 coil conversion since now I have 2 silver coils instead of the original 2 black Bosch ones. I have heard good things on that retrofit.

Ignition system is stock motronic nothing aftermarket other then Steve Wong tune.