Hilton

There's a handy page in the WSM on diagnosing hot starting issues..

Page D24-24 in LH-EZK_Diagnosis / Troubleshooting (very tail end of the Volume 1A, pg 286/307)

In my case, it was a failed check valve (which I'd assumed to be good as I'd replaced it when my fuel pump failed a few months earlier).

Other possible causes are failed Fuel Pressure Regulator, or leaking injectors.

When the engine's cold, the fuel pump can deliver pressurised fuel pretty quickly to the cylinders - but when the engine's hot, the system vapor-locks as the hot fuel vaporises in the lines.

Having a fuel pressure gauge is needed unless the FPR has obviously got fuel in its vac line - in my case the drop in pressure with a non-working check-valve was instant; as soon as the engine stopped. Verified by clamping the line from pump back to the tank and it held pressure.

the flyin' scotsman

The test plan is on CD1 of the Jim Moorehouse collection.

Andrew.......is you need a FP pressure valve I've a new one on a fuel pump I just replaced...........yours for the stamp.

Tails

Landseer,
Malcome has identified the correct location on Jim's Tech CDs.
By using this method it will stop the "jumping all over the place" syndrome.
Tails 1990 928S4 Auto

AO

Update...

First off, I know how to diagnose this stuff and have contributed to other people's hard-start threads. I know where the test procedure is on the manuals. The purpose of this exercise was to hopefully get newer members to scratch their brains as if this was their problem. The old guard here is very good at what they do. I thought it would be fun to bring up some new recruits.

OK so last night I went to the car and measured the temp II resistance while cold. measurements were taken between the leads and the body of the Temp II sensor. Values were 2.49k Ohm and 2.48k Ohm. Right within spec.

I started the car and ran it for 10+ minutes. Shut it down and could NOT reproduce the problem!

So I did some more probing... and may have found the problem. Resistance between the water bridge bolts and the chassis was about 6 Ohms. But between the Temp II housing and the chassis, resistance was 148 Ohms (temp I was about 100 Ohms). Not sure if this is enough to throw the LH into a tizzy or not, but it appears I need to either put a ground wire to the Temp I & II sensors or clean the mating surfaces/threads of the water bridge. I'm leaning toward a ground wire for now to see if that eliminates the problem. If it does, then I do a permanent fix. If not I'll investigate other potential causes.

Landseer

Thanks for the update. If you were to utilize the shark tuner, would you be logging the various sensor signals, then manually scanning logs for variations?

AO

The Shark tuner would show all kinds of values and would clue me in on certain things to check. Could be Temp II, TPS, etc. If they were all in spec/functioning, I would then investigate other items like the fuel pressure regulator, Dampner.

One other thing I neglected to add to my update... As the car was running I was looking at the sparkplug wires (They are brand new Beru). None of them were fully seated on the plugs. I don't know why I didn't catch this before. They were all flush with the cam covers instead of slightly recessed - probably becasue they are brand new. I could not push them down by hand and had to use a light mallet to tap them home. While I doubt this was the main cause, it could be a contributor.

Tony

I had a whole bunch of little gremlins going on at that time but that was the reason for an intermittent cut out...not there anymore. My no start when hot/warm was a bad tune on my ST'er. Some how and at sometime i must have inadvertently saved something and my cranking map was all out of wack! Starts great now though..still a bit on the rich side, but starts!

the flyin' scotsman

Given the measured resistances perhaps we should check a good running car for comparo?

Also, I too have noticed slightly unseated spark plug caps...............I wonder is the engine expansion is enough to cause lift? Never tried the mallet trick though

jon928se

Nah - manual choke pulled out to hang his handbag (purse) on it.

the flyin' scotsman

ok............I verified the resistance values at the locations Andrew measured on my '88 S4:

both positions measured 0 ohms.

Now where's that mallet?

PRC928

Hey Andrew,
I am sort of a new guy scratching my brain. Just had an internittent, super rich idle problem with a friends '90 S4 that turned out to be the LH. Do you think that could cause this problem?

AO

Malcom, thanks for confirming. I was just about to do the same today on my GT.

Paul-
Probably not a problem with LH. The only problem this car is exhibiting is hard hot starts. Once it's running, it's fine. A LH failure often times results in injectors rapidly clicking - that is not happening on this car. If I exhaust all other options, I have a spare LH should that be the problem.

AO

I owe you guys an update on this issue. I experimented one day when the idel was erratic. I bridge the body of the Temp II sensor to the engine block. Idle immediately smoothed out. A ha! You see i had the water bridge powdercoated, but I didn't strip the coating off the mating surfaces as had been suggested by other. I thought they were suggesting that to get a better seal - not better grounding.

So I made a mini ground strap to ground the temp II sensor to the water bridge bolt. Voilla! Engine purrs like a kitten and no more flodding when trying to start.

Thanks everyone.

Mongo

What does your mini-strap look like? I'm willing to try anything for my idle prob.

AO

A pice of wire wrapped around the body of the Temp II under the washer with the other end on the water bridge bolt, but you could attach it virtually anywhere.