928s4forme

Hey Everyone! This is my first post so please bear with me...

I just did an engine swap on an 87 S4 with an 88 S4. The donor car was a total loss rear end with a build date 4 months older than mine. Everything went well. Then the electrical gremlins hatched. Driving down the road one day on the way home from dinner with my wife in the car behind me and all of a sudden dead...Pulled over cranked it and no luck. Called AAA and prayed for a flat bed. Waited 45 minutes (live in the NC mountains) and tried it again...she fired right up! Drove another 4 miles...dead again! Cranked her a few times and she wouldn't hold idle...so I two footed her the rest of the way home. Here is what I've done so far:

-Verified fuel pump works by bridging the relay
-Swapped brains with the 88 donor car (donor worked perfectly before swap)
-Plugs are dry
-When she's cold she starts up and runs perfect
-Temp sensors are working within spec based on temp
-When she's warm she won't hold idle and searches for idle
-Cooling flaps (up front) constantly cycle after she stalls (have to pull fuse 23)
-New Battery
-Hard starting when she is warm and starts perfectly when she is cold
-Idles perfect when cold and holds idle even after rev up when cold
-No fuel residue at dampers or regulator
-Will be checking vacuum systems (both) tomorrow (AC flaps work ok in cabin)
-Using 93 octane fuel
-CPS sensor will be replaced by Sunday

Could use some serious help

namasgt

that usually happen when you have trust bearing failure....
search for "trust bearing failure" and for "flex plate release". It is possible that the donor car had this issue. Flex plate needs to be released, how many miles, I don't know. But a lot of bigger engine 5 liter 928s have been junked due to this issue since trust bearing failure usually involves engine block damage. Check your oil for metal chips.

namasgt

How many miles do you have on your new install, The above could be the case if your car is an automatic, or if the donor car was an automatic.

fraggle

Dry plugs and sketchy flaps sounds electrical to me. Are you sure the temp sensor harnesses are all ok?

928s4forme

I have about 150 miles since the swap. I checked the flex plate and movement was nominal. I agree, these are similar TBF symptoms, but when I checked the oil, there is no evidence of metal or "glitter". I'm with you Fraggle, it appears to be electrical in nature. I have swapped the CE panel when I did the brain swap and replaced all the relays and fuses. I have cleaned some of the MPs, but not all the them. I will double check the harness (that is a good place for it to short to ground and throw the idle stabilizer off and cause the LH to try to compensate. It's almost as if when the system goes from open to closed loop (thinking of K-Jet technology instead of LH) it's not compensating for the change in temp and causing it to run rich, hence the idle stagger. I will let you know about the harness. I am looking for a short or short to ground with all the harness plugs disconnected, correct?

namasgt

That's good news. Did you install the engine ground strap tightly on the passenger side under the car when you put the new engine in? I know John had similar issue with his car quitting after a few miles, found out engine ground strap was loose.

Have you tried another MAF sensor? You can have yours rebuilt if that is the issue.

928s4forme

Funny you asked about the engine strap on the passenger side. I thought about that yesterday and jacked the car up just to clean the ends and retighten the engine ground on both the block and the body. I've got a spare MAF and will swap it out before I change out the CPS since I am going to be "in the neighborhood". Thanks for the helpful advice. I'll keep everyone posted as I move forward trying to slay this beast...

soontobered84

My main engine ground was not loose. It was tight, but a thin layer of corrosion prevented a good ground and created an intermittent ground. It ran great when the engine found the ground and would not run when it did not.

Podguy

You put an 87 control panel in and 88? I think they are different. If I remember the 88 is a one year only.

fraggle

Yes check carefully the 87 and 88 panels have a couple relays in different locations, including the kickdown relay.

fraggle

I'm almost positive that your problem lies in the panel somewhere. When I had a "normal" relay in the kickdown location on my 5 speed, it would randomly short the cps signal to ground - but only after the car warmed up.

928s4forme

the CE panel for 87-88 model years were the same. 89-90-91 were all one year only.

here's a supporting link: https://www.928gt.com/t-89fuse.aspx

Laying in bed last night, a thought occured to me: What about the rubber hoses in the intake tunnel? Don't they affect compression and idle stability in closed loop operation? What if I'm losing compression when the system goes closed loop after warm up and switches from rich to lean mix and enforcing idle stabilization through the electronically controlled stabilizer? Hence, the inability to maintain idle when hot, but revs fine and runs with the hammer down... Just a thought. What do you all think? Will be looking into this later tonight...

By the way, I cranked her up this morning and she turned over perfectly...
She is such a tease (reminds me of a girl I knew in high school)...

GregBBRD

Crank trigger and water temperature sender (for the injection, not for the gauge) would be where I would go, first.

High resistance on the main coil wires leading to the caps will make the spark jump over to the trigger wires from the computer. Those black plastic covers over the coils keep water out, but they also keep the coil wires from plugging into the coils, completely. Any resistance on that wire will "leak" around the boot on the coil wire. This high voltage "shock" backfed into the computer will shut the engine off. Make sure that the coil wires, caps, and coil have no "green" corrosion growing inside. "Trim" the plastic covers until the coil wire boot gets completely onto the coil.

There's also an aftermarket set of spark plug wires (no names) that have a "push-on" connection at the coil end, instead of a "screw-on" connection. Common for this connection to come loose....under the boot, where you can't see it. I've seen this resistance (from only one wire "jumping" spark make the coil ground out to the trigger wire from the computer) shut an engine off, more than once.

"Path of least resistance stuff."

The "first one" was a real bitch to trace, BTW.

928s4forme

I will double check to make sure the ends are seated and that I don't have any high resistance connections from the coils and/or dizzy covers to the plugs. Also, I'll check for "fireflies" tonight after it gets dark...see if I see any "blue trails" looking for ground. Thanks for the advice guys...I'm getting a lot of good advice here! Will give you a sitrep when I can...

outbackgeorgia

928S4forme,
I bring my S4 up to the NC Mountains frequently to run the BRP from Maggie to AVL. Where are you located? Will be up again in a few weeks.
I do know where all the ground connections are!
I can bring up the relevant tech manual elect diagrams if needed.
Seems like electrical issues to me also, as Greg said, can be really elusive. Connectors crumble to dust!
Dave
Dave