marknsf

Thanks in advance. I’m trying to determine what could be causing an intermittent ‘no start” problem on my 928.

About 5 months ago I acquired an ’87 S4 with 95K mile. This is my 3rd Porsche, I had a ’74 911 and still have a ’86 944 Turbo. Since acquiring the 928 I have been attacking various pieces of deferred maintenance.

A little over a weak ago I completed a TB/WP refresh along with some WIWIT items including cleaning various ground points with DeoxIT. Thanks to the guidance and hands-on help of Bill Ball things when smoothly. The day after completing the job I drove to a store 20 miles away. When I came out the car wouldn’t start. The battery and starter are stronger and the engine turned over fine but no start. As I far I know this problem had never happen before.

Since Bill lived nearby I gave him a call and he came over. He suggested changing the fuel pump relay. I had recently purchased 4 #53 relays so I disconnected the battery, replaced the relay and reconnected the battery. We pulled a spark plug before starting and observed it was dry. The car started with no problem.

Back at home I opened the relay and found the contacts were severely pitted which indicated to me that Porsche had not properly protected the contacts from arcing caused by the back EMF voltage from the pump motors when the contacts open. Not surprising this relay is a common failure. Still the contact resistance was less than 0.1 ohms and I was somewhat surprised it was the problem.

Over the weekend I replaced the other critical relays and started the car on various occasions without problem. This evening I removed the spare tire and disconnected the battery to clean the ground contacts in the spare tire well. As with the other ground points the connections were in good shape with no corrosion. This is a southern California car that has always been garaged so there is almost no corrosion. After reconnecting the battery the car was back to “no start". I tried multiple times over several hours with the same result. One time I saw the tach bounce up to a 1000 but that was all.

Having spent most of my working life in software development I knew that intermittents require a very systematic approach. I confirmed that the 3rd party alarm only disabled the starter so it was not the problem. Next I disconnected the battery and then reconnected it with no other changes. The car started up with only a slight delay, which I presume was to build up fuel pressure.

I repeated disconnecting and reconnecting the battery several times with no further "no starts."

So disconnecting and reconnection the battery clears the “no start” but what is the real problem? My thoughts are that relays and sensors shouldn’t care about disconnecting the battery since they are not powered when the key is off.

I believe the computers are original, so could it be a computer problem? I've read that the hybrid circuit eventually fails on the LH but does it fail intermittently?

What might cause this type of intermittent no start problem? How can I verify the cause?

Any thoughts appreciated.

Mark

Mrmerlin

has the CE panel been sprayed both sides with deoxit?
any car is susceptible to blower box leaking,
if you see corrosion on the bolts above the CE panel then yours is leaking.
I would consider swapping in a new ground strap,
making sure that you have power going out of the fuel pump fuse wire, though the deoxit should make this connection stronger.

make sure that the fuse blades also fit snugly.

Based on your info its an electric issue and you have not fixed the weak link yet.

Also spray the computer connections and consider sending off the computers for a preemptive rebuild

SteveG

There are several other grounds that can cause an intermittent. In the New Visitor thread there is a diagram. That pic will only point you in the right direction or location. Look for brown wires. It's prob not a coil issue, but those wires and the coil grounds should be cleaned.

Mrmerlin

the usual no start grounds are above the CE panel,
and the back of the engine V under the CPS connector

WallyP

Sometimes we get stuck with doing no-start trouble-shooting in the dark in a parking lot...

If you have someone who can turn the key, this might help on an LH-equipped car:
Kneel at the rear bumper and have the helper hit the starter. Listen for the fuel pump buzz, then sniff the car's butt (sniff the exhaust for a gasoline odor).

If there is no fuel pump buzz, the problem is usually the fuel pump fuse or relay.
If there is a fuel pump buzz, but no fuel odor at the exhaust, the problem may be the injection relay or the Temp II sensor (on a cold start) or even an empty fuel tank.
If there is a fuel odor at the exhaust, the problem may be ignition, crank position sensor or the Temp II sensor (on a warm start).

Won't always help, but it's fast, free, and makes it look as if you know what you are doing.

WallyP

OK, just had a strange experience. I posted the above answer, and upon reading my post, I realized that I have omitted the crank position sensor as a possible cause for a no-start. I hit the edit button and added "...the crank position sensor..." to the post.

When the edited post appeared, I noticed that "crank position sensor" was in a different font and color, as in a URL link. I clicked on the link, and the Advance Auto website popped up with a page of crank position sensors! When I backed out of the Advance page and went back to my edited post, the link was gone...

I buy quite a bit from Advance, and have an account there, but it is a bit spooky to realize that they apparently are watching everything that I do! I think that I will see if I can stop that.

Edit: Just happened again with this post, but on the original, not after editing.
ReEdit: My son logged in as a visitor, and the links show up for him as well in both Chrome and Firefox - looks like it is this forum. I'm not too happy with that.

SteveG

OT : Wally, FYI, I have cookies enabled, but no browser history is recorded, at least not visible to a lay person like me. I don't see any "effects" or link in your post at this time.

John Speake

Every time you disconnect the battery you clear any adaptation parameters that are stored in LH and EZK ECU is SRAMs.

What's the history on the MAF ? If it is well aged then the adaption will be driven crazy trying to compensate for it's screwy output readings... just a thought.

marknsf

Thanks to everyone for their input. I worked on various suggestions today and tried starting/running repeatedly under various conditions, it started right up each time. It’s always hardest to fix a problem when it isn’t there.

I had cleaned most of the grounds during the TP/WP procedure. After the first no start I cleaned the ones above the CE panel and in the spare tire well, they showed no sign of any water or other corrosion. While replacing the critical #53 relays I sprayed each of their sockets with DeoxIT.

Today, I cleaned the grounds for the coils and ABS (that are almost certainly not the cause). I tried to clean the ones at the rear of the engine but I couldn’t find a away the actually get to them so I just sprayed them. They don’t look bad but if I can figure out how I will clean them better. I replaced the Fuel Pump fuse and sprayed the fuse socket contacts. I also disconnected and sprayed the LH and EZK connectors. They looked fine. I did find the contacts on the temperature sending unit (not Temp II sensor) were a little lose so I cleaned, sprayed and tightened them.

I added a cutoff switch to the battery ground which makes it easier to disconnect the battery when needed.

I searched forum and found a 5 page 2009 threat started by Eturbo924 that described a similar problem. Unfortunately, the thread ends with him about to try a substitute LH. He never came back to say what the solution was.

https://rennlist.com/forums/928-foru...-question.html

John thanks for your comments, as you say the parameters may be confused but could they cause a no start? I assumed they would just get reset to a default, but I don’t know how the ECUs really work. BTW, you mentioned in the above thread that LHs almost never fail intermittently.

The PO replaced the MAF about 3 years and 5k miles ago after the car failed California smog so it should be ok.

For now I will continue to check/clean things and try to reproduce the failure. If I can then I will try some of the suggested diagnostics.

I’m still suspicious of an LH (or EZK) problem because nothing else should have been effected by the battery disconnect.

Thanks again for the suggestions and if I learn anything new I will post it here.

Mark

outbackgeorgia

On my 87 it was the crank position sensor connector. It literally crumbled and caused intermittent starting and running.
Located on the drivers side under the air intake. Also check the MAF connector. They tend to crumble also, but less common. I suspect you will ned to have the LH computer rebuilt replacing the hybrid circuit. It can cause the symptoms you describe.Fix it now or fix it later, it will fail, probably not in your choice of location (how about I75 south of Atlanta on a hot July day).
Good luck

John Speake

Yes it does seem unlikely screwed adaptation would give a no-start...

When you disconnected/reconnected the battery did you check the two smaller red wires on the +ve terminal that are held with a separate bolt to the main +ve clamp ?

AO

It sure sounds like a Relay Relay Relay problem. We just saw that a cheap Chinese relay cause a no-start on an 89. Looked good, but if you twisted the relay in-place, it would fire up. Weird.

I guess the quick think to help isolate the issue is to have a spare spark plug handy and when it rears it's ugly head, see if it is a spark or a fuel problem. Use the spare plug, grab the most convenient plug wire, ground the plug to the cross bar, and see if you get spark. If you have spark, then you know it;s fuel related. Then it's a matter of tracking it down.

In my signature, I have the LH/EZK diagnostic link. Follow that if you haven;t already.

worf928

This is the key. Regardless of whether it is transient or not you (the OP) need to figure out if you have no spark or no fuel.

OP, did you clean the ground points at the back of the block?

Alan

Do you still have a stock alarm installed? - the stock alarm disables the EKZ relay.

Alan

dr bob

^^^ This first. More than a few times, loose/dirty/etc. secondary connections at the battery positive give intermittent operation of engine, and cooling fans on S4+ cars. Fuel pump, injectors, brain functions, oxy sensor heating, and on S4+ cars the cooling fan power, are fed directly by those smaller red wires at the battery + terminal. They are easy to disturb while servicing the battery, and are easy to clean and secure whenever the battery is disconnected in the well. A cheap brass 'toothbrush' style detailing brush and a 10mm wrench make the task fast and easy.