Mike J

Well the weekend started off well and then went a bit sour.

I picked up my Targa from the body shop on Friday...all the paintwork is complete, I have finished the external installation of the Targa top (seals, etc.) and the car is looking great...nice flat paint (part of Boris's (the body shop guy) standard service is wet sanding is polishing).

Anyways I still have no interior but am starting to drive the car to get some mileage on the engine.....it ran well for the first 30 miles or so and then I started to get a weird symtom : the car started to have a surge at medium throttle. Idle and low throttle were fine and a close to WOT throttle were fine but under gentle load the engine has this wierd surge...almost like a car were someone had crappy clutch work. It also exhibited the same surging on deacceleration.

In this state the car was quite driveable but then it got worse.....it decayed so it sound like the engine was running on 5 cylinders. The CEL popped up and the idle was rough. I was very close to home so I put the car in the shop. It was late so I decided to look at it in the morning.

Next morning I fire up the car, it idles fine, and go for a spin around the block. The gentle surge was back but it was running on all 6 cylinders fine. I had the car semi-warm and parked it in my driveway while I ran some errands. i came back in 20 mintues and started up the car...5 cylinders again!

OK, back in the shop and here is what I have tested so far with the car exibiting a rough idle...

1 - The CEL indicates misfires on Piston 6. I cleared it and after a while got another with the same code.

2 - I confirmed the belt was OK in the distributor. I also confirmed that pulling the piston 1 leads off the distibutors caused the car to run rougher (on both distributors). Pulling piston 6 leads on both distributors had no effect.

3 - Pulling the plug on the fuel injectors on pistons 1 - 5 caused a change in idle. Pulling the plug on piston 6 injector did not change the idle.

4 - Suspecting a bad injector I swaped injectors for piston 5 and 6. Pulling the plug on piston 6 did not change the idle. Pulling the plug on piston 5 changed the idle. I concluded the injector on piston 6 (now on 5) was OK.

5 - I pulled the plug on piston 6 and 2 and did a compression test. Both pistons registered 160 psi (the important fact that they are the same). The rings therefore appear OK in piston 6.

6 - The sparkplug on 6 appreared to be a bit wet as compared to piston 2 but they are very close. They are both blackish and not grey but we are talking about an engine that is still in break-in cycle afterall.

7 - The vacuum is quite good reading it from the Varioram manifold.

8 - I tried to measure the fuel pressure but do not have the adaptor to push in the pessure ball in my kit. However there appears to be pretty good pressure when I relieved it when changing the injectors around.

So, what is causing the misfire on what appears to be piston 6? I have a few theories to throw out:

- I have a vacuum leak on piston 6, either a cracked plenum from the varioram to the head or something else leaking. This does not explain why there would be unburnt fuel on the plugs since a leak would be unregulated air. The idle is also close to normal.

- The electrical connection to the injector 6 is bad. I have not measured anything yet so this is total theory. However, with plug 6 being slightly wet implies the injector is shooting fuel..

- Something in the ignition circuit for piston 6 is screwed. The coils are firing fine and I get lots of spark when I pulled off the leads for piston 6 from the cap. I can't figure out how the ignition can be selective inless its the plug wires. They are quite new and I have not tested the spark at the end of the wires. That is one of the next items.

- Something internal for Piston 6 is not working. The compression is good so it sounds like its not rings. I do nto have a leakdown tester to check the valves.This car was running fine for the first 120 miles of the new engine and then it started to surge and then finally its running on 5 cylinders.

I am looking for ideas....any other ideas or places to test?

And will my problems never end........

Cheers,

MIke

SwayBar

Did you try the easy route by installing a new spark-plug?

Bull

Spark plug comes to my mind also. Then a connector that is being affected by heat?

tam993

if you suspect the plugs are OK.
I don't know if you can do this on these cars but if you have another spark plug pull the lead on one of the #6 wires and stick an old spark plug in it, ground it and start the car, see if the spark is good and it's not missing....??? at least then you know the high tension side is good, and the low tension as well...?
wonder if you could do the same with an inductive timing light? I could try that.
if you suspect a vacuum leak you might be able to find it with some spray cleaner.

Mike J

I had planned for both the timing light method and plug to ground method method.

One of the interesting things is, since there are two plugs, what are the chances that both plugs would fail at the same time? I know the distributor is throwing lots of spark, if I remove #6 plug lead from the distributor (say on both distributors) you can see the sparks arching from the post pin in the cap to the cap surface. Nice fat sparks like its supposed to have. I have not tested the other end yet, that is for tonight. And you know how easy it is to get to the upper plug of #6!

I have a few spare plugs around (I think ) and will try swapping those....

Cheers,

Mike

Terry Adams

If you find it is not electrical, could it be a valve not seating properly?

Red rooster

How about putting a 5 watt 12v bulb up No 6 injector plug. The fuel on the plug could just be carried from the other cylinders. At least you would know the DME injector drive is there ?
Good luck

Geoff

SwayBar

I would not think so since he reported good compression.

delanobe

Sorry to hear this Mike, I hope it turns out good!

Regards

Adrienne

Mike - I hope not to send you on a wild goose chase, but I had a similar problem, and it turned out to be the positive battery lead. It's a rare but not unknown problem with 993s. My symptoms included intermittent hesitation at mid throttle between 2.5-4k rpm. CEL came on with misfire on cylinder 6. We started at the basics and worked our way up, starting at the plugs, ignition wires, injectors, then lifters, ECU connections, and ECU. Nope. My mechanic talked to lots of shops around the country and apparently the positive battery lead can be problematic intermittenly. We finally resolved it by cutting the battery lead a few inches above the connection and re-soldering the lead. No problems since. If you think this might be your car's problem, PM me and I can get you in contact with my mechanic. I'm sure he would be happy to talk with you about it.

the_buch

Mike ... I'm hoping for Adrienne's solution ... and I'm looking forward to the 15th ... good luck with this problem ...

Terry Adams

- Something internal for Piston 6 is not working. The compression is good so it sounds like its not rings. I do nto have a leakdown tester to check the valves.

Mike J

Thanks Adrienne! This is an easy thing to try and I am certainly game! I will take a look at it tonight....

Oh, what did the hestiation feel like? Signficant, small? In my case the engine still puts out power, its like the throttle being dropped a bit or the engine choking a bit. It still pulls but drags a bit. Higher RPM's (as high as I want to go on the new engine) seem smoother. i also have a rough idle...did you have that?

Cheers,

Mike


Cheers,

Mike

Mike J

Just another data point. I was too wiped from work today to do much but I did manage to take the car out on a cold start. When the motor is cold to starting to warm up it runs perfect...no hesitation and fires on all cylinders. I then parked the car and let it heat soak. Almost an hour later I jump in and it missing on cylinder 6 again (perhaps a partial mix although disconnecting the #6 injector did not appear to affect the engine )

That was a far as I got tonight...new to go to bed. But the data point is that its heat sensitive. I will try another cold start tomorrow and see if it runs fine again.

Cheers,

Mike

C4S_fan

I wish I could pitch in with something meaningful Mike, but just my confidence that you'll figure it out and add it to the list of incredible stories

Keep us up to date!