AA996

I bought a 1990 S4 in October, while driving it from Michigan to Florida I had a few episodes where the car would cut off for 3-4 seconds before coming to life again and zooming back to speed (I had the cruise control on). Later, the engine would cut off completely and I’d have to pull to the side of the road. It would not start until approximately 10 minutes, when it would start and run like nothing was ever wrong. This happened three times during the day, the last time I took a long break and got something to eat. That was late afternoon, I drove until 10 o’clock that night, no more issues that night or the next morning. About 30 minutes after I got home I gave my wife a ride and a couple miles from the house it cut off again. Again, 10 minutes later it fired up and ran perfectly. I have searched this forum and Google, the closest thing I found was some gentleman had a similar problem and it turned out to be his CPS sensor. That looks like a bear to change so before I tackle that job I’m asking you folks what else could be causing this. My guess Is something’s getting heat soaked and causing an open circuit before cooling down. But what?

worf928

CPS is very likely. They have a lifetime of 15-years +/-5. So, at this point your '90 should be on its third CPS.

If you have a timing light, or some other means of checking for spark, the next time it dies on you and won't fire, use your spark-checking tool to see if you are getting spark when it won't start.

If the answer is NO then it is the CPS without a doubt.

In the meantime, check the tightness of the battery terminals and the body-end of the battery ground strap. And on the positive terminal examine the two or three small(-ish) auxiliary leads. Two of the leads feed - directly - the relays that operate the engine computers and fuel pump. If they are loose, or poorly connected then that could contribute.

There is a bespoke 928 positive battery terminal that is a 'wear item' (over 30 years what isn't a wear item) and it usually gets cheaply replaced with a simple bolt and nut with the ring terminals for the auxiliary leads just threaded around it and thus not making great contact.

This is the correct setup (for an '87; your '90 has three leads):

GregBBRD

Besides what Dave says above.^^^^^:

There's a fuel pump inside the fuel tank and a pump on the outside. The pump in the tank is there to ensure that the main pressure pump (external) always gets fuel fed to it and to ensure that vapor lock does not occur.
Very common failure item (and expensive.)
Your problem not occurring in the evening and in the morning (but in the daytime) makes me think about fuel temperatures and vapor lock.
There's a "fuel cooler" in the fuel system, which cools down the fuel by running it though a canister that is cooled by the A/C system. If the A/C system is not working, vapor lock is more common, also.
The 10 minute "cool down" also points towards vapor lock being a possible cause.
(Along with a bunch of other possible things.)

There's a bit of a "pecking order" in fixing 928 S4's that quit intermittently.
The first step, for me, is to replace the LH relay, the fuel pump relay, and the fuel pump fuse (making sure the contacts on the fuse are clean and bright.)
The relays are common enough failure items that I encourage people (who are capable of replacing a relay) to carry spares in the glove compartment. (Same relays, BTW.)

AA996

Thanks Dave, I read that if the CPS is bad the tach would register zero rpm even as the engine is spinning down. Unfortunately I didn’t know that at the time and didn’t look for it. I was busy trying to avoid semi trucks as I was coasting to the side of I-75!! I’m really hoping it’s not the CPS even though as you say it’s probably due for a change.

I have replaced the battery ground strap and the engine ground looks great as does the battery positive terminal. I’ve gone through most of the grounds on the chassis so far and have inspected and cleaned with Deoxit every fuse and housing in the EC. All looks good.

Thanks again!

Zirconocene

For what it's worth, changing the CPS is not a terrible job in and of itself. Corrosion and difficulty getting it out might cause some problems but it's not so bad if that's not going on. I was really worried about it, myself, but it was not an issue on my car.

Now, I still haven't hunted down what's causing the shutdowns on my car, but Greg's tip of relay checking and replacing is my next step(s). You're in the right place for solid advice and you have two of the best, most informed people on the planet, above, helping you out. You're in good hands.

Good luck

AA996

Thanks Greg, I have a handful of new relays in the garage - but not in my glove compartment! I’ll do as you suggest and I’ll throw a couple relays in the car too! Thank you!

Update: Greg, I was reading in the forum archives about the in-tank pump, keep in mind my 928 has a Murf Stage III supercharger, and a Bosch 044 fuel pump. I have indeed heard the pump being abnormally loud, especially after a long drive. I think you may be on to something here…

DHS928

Even while your looking for solutions start putting PB Blaster on the CPS every night until you start to replace it. With the red straw on the can and a flash light you can see it from the side and take the long shot to blast it or just pull the air cleaner and shoot down on it.. Also on eBay i found some 40 amp relays that have clear cases and a LED inside them so you can tell their powered.

DHS928

The relays car on.



LED relays.

Donald Smith

My guess, would be a relay. I picked up an S4 last year(o.k., an Audi, but still) and drove it home from Denver to Montreal. The car was cutting out, every 2 hours. The fuel pump relay was brand new, and American made, but still defective, so you never know. It was heating up with use, and situated under the rear seat, so getting to it, to test was difficult. The PO never came upon the problem, as he was flipping the car, and never drove it for 2 hours straight.
GL.

Speedtoys

https://heielectric.com/shop/electri...ght-indicator/

Don Carter

I had a similar problem on my 86.5 years ago on a roadtrip from Houston to Phoenix with my son. The car would shut down after many hours of driving (8+), then start back up after resting 15 min or so. We ended up at Dan's shop in Phoenix since he had a new fuel pump on the shelf. His theory was that my fuel pump was going out, and pulling lots of amps, gradually heating all the fuel in the tank and eventually vapor locking. New fuel pump did fix the problem and we made it home with no issues.

worf928

Interesting. (I believe you. I’ve not yet encountered a fuel pump that has that failure mode. In my world they have always been either dead or alive and 9/10ths of the dead ones have a split hose on the in-tank pump feeding them debris until they seize.)

worf928

Diagnosis over the internet is hard enough without also the assumption of psychic knowledge of modifications.

Put that in your signature or make sure it’s in your first post on a help request thread.

I suspect now you have two problems: a dying-while-running problem and a vapor lock problem because the fuel system doesn’t hold pressure once the motor stops turning.

My bet is that you have a dying CPS and a problem with the check valves in the fuel system. I’m betting this way since your service records are a bit hard to see at this distance.

AA996

Sorry Dave, you’re right. It’s going in my signature now. Thanks. I have ordered a new CPS and I’ll check out the fuel system.

worf928

How long have you owned your '90 while it was equipped with the CPS currently installed?

Do you have any idea how old is the CPS?

If the answers are "10+ years" or "no" then you will not be wasting money by changing the CPS.

You can gain insight into its status by examining the sender-side harness plug. It should be retained in a bracket on the front-side of the throttle cable pulley console. Use an inspection mirror and look for cracks in the connector housing. If you see cracking/disintegration in the sender-side connector housing or the sender-side conductor insulation then the CPS is due for replacement.

If your CPS connector isn't in the bracket on the pulley console then it has likely been replaced at least once. And inspection for cracking is easier.

The CPS signal is very noisy. The conductors for the signal must be well insulated from electrical noise. If they are not the EZK will not be able to compute an accurate engine speed. That will cause incorrect fueling and spark timing. In the worse case - where the signal is intermittent - the EZK can "decide" that the engine isn't turning and turn-off the fuel pump relay. The result is that the motor stops exactly as if you'd turned the key off.

One diagnostic you can run on the CPS is to wrap a ~16" length of mechanic's wire (I use brass safety wire) around the sender-side lead near the plug connection. Leave enough wire "sticking out" so that you can tug on the lead while the engine is running. If with the engine running, you can detect a change in idle (or the engine dies) when you push/pull/wiggle the wire then you know you have a CPS signal problem for sure.