FredR

Rob,

The fuel management system tries to keep a constant differential pressure across the fuel injectors so that when a pulse interval is set the brain has a measure of the fuel being added. The performance indication test is a gauge pressure in the rail of 3.3 barg or 48 psig when the engine is idling. thus if we assume at idle the vacuum is around minus 12 psig [for instance] the differential pressure would be 60 psi. Thus if the reference leg to the inlet manifold is disconnected the FPR will nip in to give what it thinks is the correct differential pressure and you would see 60 psig on your pressure gauge if the gauge is calibrated correctly. In such circumstance the motor would be over fuelled. Thus the first thing you need to check is that the FPR is seeing manifold vacuum as those numbers are too coincidental.

If the above is happening the motor will still run, it would be somewhat rich at idle and that may explain the smell of [unburnt] fuel - however as the throttle is opened the false enrichment would reduce and the car would run perfectly normal at full throttle but that as I understand is not the case.

That being said in my opinion the vacuum ref leg issue alone would not explain what you are seeing. If the MAF is not working and has gone into limp home mode, it would run OK on part load at low rpms over a very narrow range and as the revs were increased it would simply stumble and not rev.

928 at last

Thanks Fred, I'll check the vacuum at the FPR in a bit and report back....

928 at last

Peanut,
"Solid" in this case means no fluctuation in rpm at constant throttle, and all cylinders seem to be firing.
Attached the gauge to the front of the passenger side fuel runner at the usual connection point. Started the car and let it die twice, got out and read the gauge. (I should note there was no decrease in pressure on the rail after letting the car sit for twenty minutes.
Nothing disconnected.
I've always suspected my gauge reads a few lbs. high, and my point was that there is no material change in the fuel pressure at the rail from when the car was running well to the current situation.

FredR

That is how I interpreted your words and the clue there is that the gauge pressure should change as the throttle is opened and closed [within reason] and thus the depth of vacuum drops and then recovers.

peanut

yes force of habit I meant the fuel pump relay bypass .On a 944 its a double relay called the DME relay

I have been saying check the fuel pressure and the FPR and vacuum all along and been ignored
Now everyone is saying the same thing

worf928

Let us review some basics here...

First and foremost, 928s are hard to kill. When they deteriorate to the point of not running it is almost always the result of two or more failures.

Fuel Pressure:
On a *928* engine-off-no-vacuum-fuel-pump-relay-jumpered or all-fuel-vacuum-lines-disconnected fuel pressure should be between 52 and 58 psi (3.8 BAR +/- 0.2 BAR.)
Running, with vacuum at both dampeners and the regulator fuel pressure should be 3.3 BAR +/- 0.2 BAR (45 - 51 PSI).
Or, in other words, removal of vacuum from the fuel system will increase fuel pressure by 7 PSI.

Fuel Vacuum System:
The vacuum system as it connects to the dampeners and regulator in an LH 2.3 system has basically nothing to do with cold start.
The primary purpose of the manifold-vacuum-changed fuel pressure is to avoid tip-in detonation with large, abrupt throttle plate changes.

At start and idle, fuel pressure issues on a 928 are usually binary: you have pressure or you don't. 'Wrong pressure' issues are rare.

Common Reasons for poor start/idle/surging in no particular order
Mass-Air Sensor exhibiting uncommon (not just out-of-spec) failure mode.
No idle switch signal
LH ECU
Idle Stabilizer not, or poorly functioning.
Incorrect temp signal to ECUs

Less common:
Crank Position Sensor, harness, or connection on it's way out and providing a single too noisy for the EZK to find the correct engine speed.
Pressure dampeners, regulator diaphragms ruptured and passing unmetered fuel to air guide.
Loose LH harness grounds.
Poor ground at TEMP-II due to powder-coating or painting the water bridge.

My inspection guide (download link in my signature) contains instructions on how to test the idle/WOT switch, ISV and Temp-II sender with a multimeter. A vacuum hand-pump can be used to test integrity of the fuel pressure regulator and dampeners.

@928 at last Do these simple tests and report back.

worf928

In order to refresh my context, I just re-read the OP.
Test TEMP-II readings at ECU plugs as per my inspection guide.
Test vacuum integrity of regulator and dampeners with a vacuum hand-pump.

Questions for you:
Was the water bridge 'coated' in any way during the intake refresh.
Were the LH harness grounds at the back of the block cleaned during the intake refresh?
Did you try your fresh Mass-Air Sensor yet?
Confirm that, now, there is no condition under which the S4 idles OK.

928 at last

Confirmed. Starts and immediately dies. It does fire up immediately though with minimum cranking, so no change in the starting behaviour.

worf928

Ok.

This behavior is consistent regardless of engine temperature? Whether hot or cold it fires up and then idles like crap? Has the ambient temperature been consistent throughout this ordeal? These questions asked in order to figure out if actual engine temperature has an effect on the behavior.

Also, let's keep in mind that you may have more than one 'issue.'

What work on the S4 was done, if any, between the time it ran ok and when it started showing 'signs.'


It seems ridiculous for a TEMP-II water bridge ground issue to surface after years of use. And since the behavior is the same hot or cold, let's leave this topic for after you've tested the TEMP-II at the ECU plugs. If the TEMP-II test shows incorrect resistance values at the ECU, repeat the test at the sender to rule out harness issues.

Ok. Remove the air box and check the two ground bolts to make sure that they are tight.

See below.

Also, test the ISV as instructed in the guide.

928 at last

Will do.

928 at last

Update.....
FPR and both front and rear dampers are holding vacuum, and are receiving manifold vacuum. Manifold vacuum is difficult to gauge with any accuracy as the idle is erratic at best with my son trying to feed fuel and keep it at low revs without stalling. But, it's bouncing up as high as 20 in. Hg. so at this point I have to assume it's getting the appropriate vacuum in the system.

Went to remove the old MAF, and apparently I installed it correctly last time I was there. Can't for the life of me remember how to access the clamp to release the MAF from the boot. Any quick pointers appreciated.

FredR

Remove the passenger side fuel rail insulator and access the jubilee clip with a long screwdriver either under or over the fuel rail between cylinders 3 and 4 - a torch helps you see the screw head for engagement.

The lower the revs the deeper the vacuum you will see. My motor pulls something close to 20 inches Hg normally or so I seem to remember. If the FPR is receiving vacuum then either your pressure gauge is incorrect or something else rather bizarre is going on.

Did this problem suddenly appear after the car was previously working OK or did it start after some intervention?

928 at last

Thanks Fred!

The history is.....Car parked for the winter as usual....got it out a couple weeks back...running fine. Got it up in the air, and practiced my best "vocabulary" and replaced the high pressure steering line, son and I drove it for a week and a bit...no issues, running fine. Then took it out to run a couple of errands last Sunday, and on the second warm start-up, it surged three times and died. Second try..same. third try same....quit and called for a tow.
Later. car cold....tried again....less surging stalled more quickly. Etc. Etc.

Now with the MAF unplugged, the car will start with no one on the pedal, no surging, fires right up and then dies. With gas being fed, will fire and run (badly) and over 1,000 rpm not happy but running, over 2,000 to 4,000 runs much better but still doesn't feel or sound right. Trying to get the old MAF out at the moment and install a rebuild......

worf928

You're trying too hard. If the regulator and dampeners hold vacuum you're good. If the vacuums lines *don't* hold vacuum and are connected together at the 5-way which is connected to the air guide, then... you're good. No need to ride the bucking bronco to check for manifold vacuum right now.

Now go check the TEMP-II, ISV, and idle switch.

What Fred said.

dr bob

I'm a believer in using an extended-shaft nutdriver or a socket on extension for the MAF clamps. Once engaged, the nutdriver or socket takes a lot less effort to keep indexed on the clamp as you adjust the clamp You don't have to push as you do with a screwdriver, so the clamp has less tendency to walk away around the boot..