mossy

Hi Guys,
I hope someone can help with this, it's been bugging me for ages.

The S has been misfiring since I put the new engine in. I didn't want to post this until I was 100% sure that I'd tried everything that I could think of.

The misfire consists of an almost imperceptable hesitation accompanied by a metallic tink (very quiet) from the exhaust. It happens in all conditions under all loads. The cars performance doesn't seem to suffer too badly, and the fuel milage is about right (I have a heavy right foot, so I don't expect miracles!!!).

Previously the car had a top end faliure, so instead of repairing the head I decided to replace the whole unit (a friend had crashed his identical car with a little over 50K on the clock).

When hooking up the new unit (they are a PIG to change) I noticed that the new unit (also a 1988 944S) had a sealed off hose (looks factory) on the intake manifold, where mine had not. I don't know if this is relevent...... Everything else swapped over perfectly, the only thing not hooked up is the power steering pump sensor.

The car also seems to be running rich. The plugs are very sooty, but there is no smell of unburnt fuel from the exhaust.

So far I have checked:
1. FPR - at both ends of fuel rail - they're fine.
2. TPS - found faulty and replaced and adjusted.
3. Temp. Sensor at front of block (within recommended Ohms)
4. Checked timing as per Workshop manual (with dial guages)
5. Tested for loose connections, dry solder in engine management computer
6. Replaced fuel pump
7. Checked for vacuum leaks
8. Compression checked the engine, all are fine at around 190
9. Viewed the combustion process with Colortune (transparent spark plug) every 4th or 5th spark seems weak (orange flame as apposed to blue)
10. I have taken the car to a 'specialist' who guaranteed their diagnoses of the AFM. New AFM - no change.

I'm thinking that the problem is electrical, could something as simple as a weak battery cause these symptoms????

I'm at my wits end, I don't want to throw any more money at things that it 'could' be. It's not a major problem, it's almost imperceptable and the car is 100% reliable. I just feel that I'm missing out on that 1%.

If anybody has any ideas I will be eternally grateful........

Mossy

Campeck

maybe the distributer cap?
or coil?

mossy

I'll check the cap. Thanks.
Do you know if there's a testing procedure for the coil??

Campeck

I'm not sure.
I just though that there might be something in there interfering with the spark or its worn down If thats possible

mossy

Got to be worth a try!!

Big E

???? What the heck is that? Never heard of it, sounds like a great idea. Where do I get one?

mossy

Dunno if they still make them. It's basically a transparent spark plug that they used to use to tune carburetter jetting. You replace one of you spark plugs with it and you can see inside the combustion chamber while the car is running (pretty cool!). From this you can diagnose the temperature of the fuel being ignited by the color of the flame (hence ColorTune).
I'll have a search on the net and see if I can find out if they still make them....

SoCal Driver

Which ignition module did you use? The one in the car or from the donor car?

mossy

SoCal, it's the original module. Had no probs with the old engine......

BigE, try here....
http://www.minimania.com/web/AddedFr.../InvDetail.cfm

Jay W

My 944S had a miss under acceleration and at idle. I thought it was fuel related, but found that the distributor cap was cracked at one of the spark plug wire connections. The plug wires have a long boot and it covered up the crack making it hard to see and diagnose. The spark was still jumping across the wires even though the crack was under the boot. Also you could have a bad spark plug wire that intermittently fails.

SoCal Driver

Same coil?

I would look at the ends of the coil to dist HV wire. These tend to come loose or corrode under the weather boot. Either way it degrades the current/spark.

mossy

Jay, as soon as it get's light I'll be checking all the HT leads and distributor cap, while it's dark I'm going out now to look for 'fireworks' in the leads.

I'll post back later to update.....

p.s. it is the same coil.

mossy

Doesn't look like any of the leads are shorting.
Checking the cap in the morning.

Any other S owners experienced anything like this???

Robert D

My car has been doing some strange things as well. Somebody explained what the sealed off hose on the intake manifold was for, but as it was factory I seem to have forgotten. The problems I have are that the car does seem to miss, as well as the idle wanders occasionally. I've replaced the potentiometer on the AFM, the ISV, CAP/Rotor were done with the head, valves and guides. I bought a new TPS, and I'm waiting to put it on this weekend (hopefully it will solve the idle issue). Still no clue what the miss is from. Very very odd. I wonder if the coil may be the problem...

mossy

Thanks Robert,
Where is the Idle Stablizer valve located, and is there a method of testing it?? This is also on my list.
As soon as I find the solution for this problem, I will post it proudly in big letters. It's been bugging me for far too long......