Head scratcher...

[adam242]

My new 924S is driving me crazy... rough running issue I cannot figure out.

A little history, the car was purchased by a friend in Dallas after sitting for several years. He got it running (something about the injectors being stuck), changed the timing and balance shaft belts, a few other odds and ends and decided it needed a new home. He has too many projects and I got a great deal on it... I've always owned German cars, including a 944 a few years back, so I jumped at the chance to give a clean Texas 924S a good home.

My friend drove it 1200 miles from Dallas to Detroit and it was running fine.. averaging 28 MPG for most of the trip. The last leg (200 miles or so) it began running rough and fuel economy dropped to 24 MPG.

After it changed hands, we've been trying to diagnose the problem. Pulling the plug wires one at a time revealed that cylinder #2 is dead. The wires were in bad shape, so I replaced them.. The plugs (Bosch Platinums - number 2 was badly fouled) were replaced with Bosch Supers. No change.

I pulled all the injectors and bench cleaned/tested them per the tech article available on this site.. all fired and had a good spray pattern when I was done.

Injectors are receiving pulse from the DME... checked with a 'noid light.

Fuel pressure was found to be high (50psi at idle) so I replaced the regulator with a good used one from a BMW 528e. Pressure is correct (30 or so at idle) now.

Still no change... swapped #2 injector with another cylinder, number 2 is still dead, swapped cylinder works, so injector must be functional.

Compression is 170-175 on all 4 cylinders.

Replaced worn cap and rotor - spark is good and hot.

Verified timing marks for the cam belt.

I've checked for vacuum leaks and checked/cleaned grounds...

I have fuel, compression and spark, yet my Porsche is still running on 3 cylinders! Can anyone point me a new direction to diagnose this? I'm pulling my hair out.

[Ga 951]

Good job on the trouble shoot.

I'm certainly not the sharpest knife in the drawer, seems like the missing component is AIR. Is the intake valve opening? I think you need a leakdown test to find the prob.

Keep up the good work, you're on the right track!

[John_AZ]

See if the DME connector 35 pin wiring chart on post 4 will help. You may have a wire connection/wire short or open on the fuel injector control.
See note below.
http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/575220-no-injector-pulse.html

Note: the fuel injection control is incorrect (6 cyl shown). DME pin 14 controls cylinder 3 & 4 and DME pin 15 controls cylinder 1 & 2.

All four injectors will fire at the same time, It is called the "Motronic banked simultaneous injection system"

GL
John

[jeryst]

Probably a dumb question, but are you getting a good spark on cylinder #2?

[adam242]

Yes, cylinder two has a good spark, tested good injector, pulse to the injector, proper fuel pressure and 170 PSI of compression, yet is still dead. Pulling the plug wire or the injector connector causes no decrease in RPM or increase in roughness.

This weekend I will check the DME wiring and perform a leakdown test, thanks for the advice!

[dangerous]

How did you determine #2 was the dead cyl?

[adam242]

Originally Posted by dangerous

How did you determine #2 was the dead cyl?

See the above post.

Pulling the plug wire (for cylinder 2) or the injector connection (for cylinder 2) does not cause an increase in roughness or a decrease in RPM.

[alfadoc]

Wow. Subscribed. Can't wait to see how this turns out. You say you've got spark, and you've got fuel. Anything mechanical (wiped cam lobe, etc) would show up in a compression test. Internal combustion basics: spark, fuel, compression, all happening at the right time. Can't wait to see how this turns out.
Buon Fortuna!

[adam242]

Okay, here's the latest...

Today, I pulled the fuel rail and stuck each injector in a container, then cranked the engine over. Each injector is indeed delivering fuel. I didn't get a chance to see the spray pattern (I didn't have an assistant and I was inside the car turning the key) but I think I can assume the pattern is acceptable. I just cleaned the injectors, and I had swapped the injectors on cylinders one and two without the dead-cylinder problem moving.

I also performed a leakdown test with the engine cold, using a Harbor Freight leakage tester. Ran the test several times, and all 4 cylinders are consistently in the 15-20% range. What little air is leaking seems to be getting past the rings (hissing in the dipstick tube) and not the valves.

Still stumped!

John_AZ, is there any particular lines from the DME I should check? I *am* getting injector pulse..

[John_AZ]

Review:
Drove the car 1200 miles and it ran fine.
Last 200 miles ran rough
All cylinders are getting fuel and all injectors are firing with NOID test
Now cylinder 2 is not firing. No spark to cylinder 2.

According to the WSM electrical chart, Pin 15 on (big/main) 35pin plug controls injectors 1 & 2



From what I have learned, The DME controls/grounds the coil and sends the spark to the cylinders.

NOW,

Do some of the basic tests because the car was driven 1200 trouble free miles.
(From the WSM-work shop manual)
1. Disconnect the battery and clean all cable ends.
2. Go under dash and pull the big 35 pin connector and check for bent pins/sleeves and corrosion.
Connect the battery wires and try to start. Gently tap the DME box and wiggle the connector while trying to start and checking cyl 2 for firing (bad wire).
3. Clean all grounds, especially the large block of grounds attached to the underside of the dash on the drivers side ) above the fuses and relays.
Clean the main negative line ground on the engine and the separate tiny 2 wire ground near it.
4. A later TSB-(technical service bulletin) says to clean all the engine electrical connections on top of the engine if running rough.
5. Check the reference sensor connections and clean. Check the sensor wire for frayed sheilding and loose wire connections.
6. Check all fuses and the back of the fuse and relay panel for burnt wires, check the wires for fatigue and poor connections.

Next,
Here is a troubleshooting page from Clarks:
Troubleshooting - Engine

Here is a DME troubleshooting photo:


If still nothing ....Here is the link to Clarks DME testing and inspecting page.
DME Computer - Testing and Inpsecting

Without an oscilloscope further testing of the DME Computer cannot be done.

Now it will come down to bad/old solder joints in the DME.

Here is a link on how a member resoldered some connections---at your own risk.
DME Repaired w/pics - Rennlist Discussion Forums

These tests should help if nothing more than doing basic checks over and over until you find the problem.

You have done a lot already, good luck.
John

[adam242]

Review:
Drove the car 1200 miles and it ran fine.
Last 200 miles ran rough
All cylinders are getting fuel and all injectors are firing with NOID test
Now cylinder 2 is not firing. No spark to cylinder 2.

Just to be clear, I _do_ have spark on cylinder two.. I connected a good plug (that I later installed in the engine) in the boot and set it on the intake manifold. Good hot spark with the engine running. That's why I'm so puzzled.

Thanks for the thorough response, John. I truly appreciate the help. I'll work through the list tomorrow.

[John_AZ]

Edit:

Another 35 connector pin display:
DME - Connector 35-pin

John

[John_AZ]

Originally Posted by adam242

See the above post.

Pulling the plug wire (for cylinder 2) or the injector connection (for cylinder 2) does not cause an increase in roughness or a decrease in RPM.

Got it, misunderstood.

John

[honerboys]

being that i'm used to real old cars - have you tried a vacuum gauge - Yes i still have one though it hasn't been used in a while - we used to hook it up to the intake manifold - you can watch the gauge and it should have steady vacuum - if the gauge jumps down then back up it would indicate different conditions in the engine (stuck valves, valve not opening etc. ) - it's old school but you might give it a try - good luck

[adam242]

Just a quick update... haven't had ANY time to touch the car for a few weeks.

Today ckecked and cleaned grounds and connections... nothing. Both speed and reference sensors ohm out okay.

I connected a vacuum gauge (thanks for the suggestion, honerboys) and it has a bouncing needle - about 15-19 inches at idle, smooths out as I rev it. According to the chart, this indicates worn valve guides? Can this be bad enough to kill a cylinder?

Any thoughts? Thanks again to everyone for their help.

[mytrplseven]

Are you getting any smoke out the back?

[adam242]

Are you getting any smoke out the back?

No.

[vdubr928]

Bottom line.

Good compression.
Fuel delivery good.
Spark is occurring.

If all three of these things are verified and you are sure........ then the spark is happening at the wrong time.

It is very easy to ignite vaporized fuel. So there is none in number 2 when spark occurs.

[adam242]

If all three of these things are verified and you are sure........ then the spark is happening at the wrong time.

Agreed. But what would throw the spark timing off for just one cylinder? I've verified the timing belt is correctly installed and made sure the firing order is correct. I was thinking the speed and/or reference sensors... but I'd like to be certain before I drop the money.

[John_AZ]

Adding a guess.

Have you checked & changed the coil wire? Is it arching? Move the coil wire to a different position under the hood. Clean the contacts.

John