daveo90s4

Hi everyone - I'm seeking some guidance as to how best to diagnose the cause of this engine intermittent complete cut out. Any help much appreciated.

The Car. 1986 32 valve 5 litre S4. Auto. (engine does not have, and never has had, the ignition monitoring system)

The Symptoms. Driving along and engine is purring along perfectly. No misfires. No hesitations. Then suddenly the engine cuts out completely (on smooth road, no bumps). No warning. No spluttering or misfiring. Simply absolutely nothing. Then, without doing anything, the engine suddenly roars back into full 8 cylinder smooth running life. And this goes on and on and on.

What I have done so far. Ensured high tension coil wires are fully in place. Swapped out fuel pump relay for known good relay. Swapped out fuel pump for known good fuel pump. Checked voltage at fuel pump connections (good, about 12.8 - 13 v). Done each in succession, with test drive in between. None have made any difference.

My thoughts. Since the cut out results in zero cylinders firing, unlikely to be coils or plugs. It must (??) be something that controls the entire engine. And I am thinking electrical rather than fuel. But could well be wrong.

Questions: What is causing this? Or more likely, what steps do I need to take to identify what is causing this? (Once I know the cause, the solution will probably be self-evident).

Many thanks indeed
DaveO
Brisbane, Australia

FredR

Dave,

Good to hear from you. Trust all is well your end.

Not sure about LH2.2 systems but a friend of mine with an 87S4 had a problem in that the thing would start perfectly, run for 5 minutes and then cut out altogether. Leave it to cool down a bit and it would fire back up. Not exactly the same as your MO but not too dissimilar either. Whether or not LH 2.2 can have instantaneous temporary internal failure I am not sure but considering they are batch fired and those things as far as I know have a single line output from the LH output plug to the injectors and the line then bifurcates three times to form 8 synchronous firing events, it should be possible for an internal minor failure to occur. Also a fickle transient break in the section from LH to the first bifurcation node [in the engine bay] could conceivably crash all eight injectors instantaneously.

So, if you have acess to a spare LH 2.2 try firing with that and see if the problem modulates at all. If does not try to get access to an oscilloscop and see if you can detect the instantaneous voltage firing pulse probe across the terminals of one injector and then to test as and when it is misfiring- either dynamically or statically. Finally try wiggling the cable from the LH connector into the engine bay

mj1pate

It seems like perplexing things are never simple but some times they are.
When this occurred with my 86.5, after several tows and veering towards replacing the ezk/LH system, I found that among the two cables that attach to the positive battery post, the cable feeding the computers wasn’t securely fastened. A few turns of a crescent wrench and problem was solved.

daveo90s4

Hi Fred, How nice to hear from you and we hope you are both well. I will have a hunt around my mates and see if I can get hold of a spare ecu and go from there. More to follow...

daveo90s4

Hi Michael - good point. I had that exact same problem on my 1990 S4 some time ago. So that was one of the first things I checked on this car. All +ve battery connections are nice and tight (alas!!). Mind you - there could still be a loose connection at the central electric end of the battery-to-CE wire. More to follow.

daveo90s4

OK. A bit more info that I now have.

1. Above I said that the engine was cutting out intermittently. I am now questioning myself on that and consider it may be 'returning to idle' (despite accelerator position) rather than cutting out completely. I will test that tomorrow by immediately putting it into neutral when I lose power, pulling over, and checking to see if engine is idling or has cut out completely. I will then report back.
2. I find that I can influence the cut out (or 'return-to-idle'?) symptoms by playing with throttle position while driving. At very minimal throttle settings I can (often) induce the problem by applying more throttle. Having induced that problem I can then (generally) ameliorate the problem by backing off (to idle map??) or by applying full to almost full throttle (to WOT map??). So that, coupled with my suspicion as per 1 above, got me thinking (ha ha).
3. The owner tells me that this problem did not exist before he removed and reinstalled the inlet manifold (to address some other unrelated issue - he did tell me what but I now forget).

So, not wanting to rush into premature and flawed diagnoses, I am sort of inclined to think that this problem may have something to do with the throttle position sensor and or associated wiring (broken??) and the signal it gives to the LH ecu, or indeed (as Fred has kindly suggested) the LH ecu itself. I am very reluctant to remove the inlet manifold, but will do so if further easier investigations proof fruitless.

Keep the ideas coming!!

Cheers both and all,
DaveO
Brisbane
Australia

Mrmerlin

When was the last time you replaced the short TPS harness? They usually break off and cause bad connections right next to the test port at the top of the engine.

FredR

Hi Dave,

Good to know you are alive and kicking. Tried to contact you after we met up but did not see to connect so was a bit concerned as to whether you made it back in one piece. Had the PSD clutch lock up last December and had to change out the clutch from my spare unit and fettle the slave cylinder making one viable unit out of the two pieces I had [see my thread on this].

With respect to your pro blem other than the LH you might also take a look at the CPS. The male socket on these things has a tendency to disintegrate an d if so could easily create a weak or intermittent fault. My original CPS had this happen and I fitted a new male connectof to it and it test out fine. However I also purchased a new after market item from Brother Roger and that was very reasonably priced and works justfine. It also has a plastic outer shroud. The stock item has a stinless shroud fitted into an alloy block and if moisturer gets there it is a dead ringer for galvanic corrosion take place wherein the block acts aas the sacrifical anode and the
Al oxide formed duly forms a cement that mamkes extraction difficult and sometimes impossible. When I evamped my engine management harness I lengthened the CPS spur so that I could mount the connector on top of the bell housing rather than in the difficult to get at location under the throttle wheel . Now I can undo that connector without doing anything.

PanamaImport

I dealt with the same issue (random cutting off) and after spending a ton of money and time troubleshooting and swapping a lot of other parts, it wasn't until I bit the bullet and put in a new FOE engine harness that the problem went away. I'm sure it could be other things too, but that was my experience. Good luck!

Zirconocene

The issue I was experiencing was similar, though it sounds like it was not as severe as what you're seeing. The fix for me was new FPR and dampers.

Good luck

daveo90s4

Thanks everyone for your recent input

Mrmerlin - its not my car so I don’t know all its history, but I suspect that ‘under manifold’ TPS harness has never been replaced. Maybe it got strained when the inlet manifold job was done. I will keep that in mind

FredR - alas I must have missed your message. Yes, we all travelled back safely. Was great to be able to meet up. I have in my parts a known good used CPS from my 1990 S4, plus new bosch plugs. These in Australia seem easier to pull out than those in UK cars, for obvious reasons, but still a bugger to access. I will see if I can put my hands on a Bosch Hammer…

Panamimport - hoping not to have to buy a new rhd harness. Maybe last resort stuff

Zirconicene - maybe, but it just seems so sudden. All power on. All power lost. Doesn’t feeeeeeeel like a fpr etc problem, but who knows.

I will keep at it…

many thanks all

DaveO

Don Carter

I've been dealing with this issue for quite a while on my 86.5. For me, it's after about 30 minutes of driving it cuts out momentarily, then starts running again. From my research, there are so many things it could be it's a bit overwhelming. Subscribed....

928cs

I had the same kind of trouble with my early 1987 S4.
​​​​​​It was a bad ground at the back of the engine, near the firewall.

PanamaImport

I may have jinxed myself with my previous post ..... Went for a ride today and car felt great. After 15 miles or so, and some spirited driving, the car cut out (86.5 euro 16v, twin dizzy). Would crank and no start, so I gave it about 10 minutes to cool down, figuring it's somehow related to heat. I was then able to start it and drive back home, but it cut out several more times on the way back, occasionally stuttering. Aside from electrical, other things I should check that may be affected by heat? I don't think its vapor lock because I stopped to fuel up and it started right up.

​​​​​​Before FOE harness swap the problem was much more frequent and pronounced.

BRB-83-911SC

I second the Crank Position Sensor. When mine failed it started cutting out momentarily while driving (like turning the key off), and being a 5-speed, would fire back up on its own while rolling. It progressed to where it would stall, and not restart for several minutes, until it finally stalled and would not start at all. New CPS resolved the issue. No CPS signal will cause no spark AND no fuel, so check those while in failure mode, if failure mode lasts long enough. Another telltale would be no tach bounce when cranking while in failure mode. Also, could be the connector to the CPS - check that plug for corrosion and overall condition. Could also be many other things, but the CPS and /or connector would be suspect, I would think.