svo-ed

The car is a 1984 Euro 928. It starts up on the first or second try. In "PARK" the car runs idles smooth and is pretty responsive, however, if you try to hold the engine at an RPM above idle, the engine will only maintain the desired RPM for about 1-2 seconds and then automatically drops back down to idle, (even if you keep the gas peddle at the same position. )

This happens at any RPM (2000, 2500, 3000, 4000, etc.). The throttle responds instantly, but will not allow the engine to run at a steady RPM?

If you try to give it gas/throttle as the RPMs begin to drop, the engine will do one of the following (depending on the intial RPM):

1. Applying throttle has no effect at all.

2.As throttle is applied the engine stumbles and/or coughs. – I say "cough" because it is very much like a cough rather than the typical pop, pop, pop misfire.


This is a list of items I have fixed or replaced:
New Bosche spark plugs
New Spark plug wires
New fuel filter
New fuel pump
New TPS
New air filter
New battery ground, main flat strap at battery
New temp 2 sensor
polished all the grounds to mirror finish
New large diameter hose and "O" ring around the MAS
replaced a number of questionable vacuum lines.
Cleaned and polished all fuse holders
Replace all fuses with new

Any ideas? -- I have been chipping away at the most likely causes which have not solved the problem (though the car runs smoother!). I am thinking a bad spark coil (my car has two) or the MAF. -- I am trying to avoid continuing the trend of replacing good parts at random.

Thanks

The Forgotten On

I have the same issue with my 81 US (L jet instead of LH like yours). Just let it idle for a minute and then it wont stumble so much. People say there is a Temp I sensor that can go bad, in my car it is part of the AFM, not sure about yours but I would definitely check the resistance on the MAF to be sure it is within correct specs for your S2. Good luck.

svo-ed

I have already replaced the Temp 2 sensor. I was unaware that a temp 1 sensor would cause any problems. I can let the car run for half an hour but the problem does not go away.

I really thought I had solved the problem when I put on the new TPS. Seemed to make the most sense.

What about a spark coil? (I have two on my Euro.)

I also considered excessive resistance in the (+) wire leading to the fuel pump, but that would be pretty unlikely.

Would anyone happen to know if one of the fuel regulators could cause this strange situation?

I should also mention that the car is running a bit rich. Is this due to a week or intermittent spark (spark coil issue) or a faulty MAF?

Anyone with some insight or input would be greatly appreciated!

Thanks,
Ed

Mrmerlin

first is to restore the car to original running condition,
that will include getting the MAF rebuilt.
the timing must also be checked as well as the twin dizzy timing.

So with the engine at TDC remove the caps and check that each rotor is pointing at the respective hash mark on the dist housing.
You said you installed new ignition wires ,
make sure the coil wires are not rubbing on any part of their run,
and the coil sockets are not filled with corrosion.
also make sure the ground wires are clean everywhere.

svo-ed

Thanks for your input Mr. Merlin.

I have already purchased the twin dizzy, mini timing belt but have not yet installed it.

I will verify timing as described above.

The coil wires are not contacting any of the other wires or anything metal on the engine.

I have cleaned all grounds to mirror finish....even the hard to reach ones and the ones above and behind the fuse panel.

It is just weird the way the engine automatically drops down to idle from any RPM..and there is no way to prevent it. The car runs great otherwise? I would think that someone has experienced this at some point...I can't be the first?

Thanks again for your help.
Ed

Mrmerlin

the Euros seem to be easily affected by worn MAF units,
my 84 Euro ran like crap till the MAF was rebuilt and the dizzy timing was reset.

Check the coils for clean sockets,
and that they both have ground wires from the coil mount to the engine

tv

Good advice from Stan. You seem to be in the same boat as our new buddy in LV. I would always say switch known to be good (the only way is to see another euro running ok with them) 2 brains and 1 MAF from the good into yours and see what happens. Then go from there.


I believe there is a diagram in the manual somewhere that maps out the electrical/engine system. There aren't too many pieces in the puzzle, replace em all if need be (sensors, coils, relays, fuses but not the big 3) and make sure the wires are good in the harness even where currently covered and bunched (after the big 3 are checked).

Is TPS the Crank Position Sender?

also so the smoke test on the entire vac system

svo-ed

Thanks for chiming in TV.

The TPS is the "Throttle Positioning Sensor". The Euros have them on the passenger side of the throttle body. (I think the 86 and newer US may also use the same). I purchased my replacement TPS from Roger at 928sRus.

I replaced the CPS (crank positioning sensor) a few months back (also purchased from Roger at 928sRus).

I purchased a complete vacuum line kit from Roger but have not yet installed it.

I try to change only one thing at a time, then check for improvement. Most of the things I have replaced were probably due to be replaced anyway, but I am getting a little frustrated. (the car is not drivable)

By "smoke test" do you mean check for leaks? -- Could I just spray water?

Thanks for your help.
Ed

tv

The TPS, yes that black box on the side of the TB, ok, I know about everything just not big on names, i usually say that thing. I have an 85 Euro and have solved most problems that people have had on it years ago.

The smoke test is described in the manuals, cap the top of the MAF with an air proof membrane, I used half of a basketball with a clamp to secure it, then pump thru a hole in that smoke filled air under pressure, if you see smoke come out from somewhere or hear a hiss, there's your leak. (also look around that big round thing near the brake area.)

Spark plug wires correct? 1 to 1, 2 to 2, ....... no arcing in the dark?

The 3 big items are where to start, so many connections/functions there and hopefully this was not screwed with when imported.


And was this euro previously operating smoothly and strong?

John Speake

It sounds almost that the engine is in limp home mode. Try the MAF checks from my website...
http://www.jdsporsche.com/faq.html

To save you the trouble, here's the relevant piece of text...
"You can make some basic checks on the 6 way lead going to the MAF, and on the MAF itself. This will help eliminate other possible faults. You will need a high impedance voltmeter and ohmmeter :

1.To check that the MAF is being powered up :-The engine should run with the MAF disconnected, so measure for battery voltage between pin 2 (positive) and pin 4 (negative) of the disconnected flying lead. The pin numbers are not shown on the flying lead connector, but the corresponding numbers are shown on the connector on the body of the MAF. The voltage should be battery voltage (about 13v). No voltage would indicate a fault with relay XXV (relay XVI on '84 Euros)
2.To check the sensor resistor is OK:- connect an ohmmeter between pins 3 and 5 of the disconnected MAF, the reading should be approx. 4 ohms.
3.To check the idle mixture pot is OK, measure with the ohmmeter between pins 3 and 6 of the disconnected MAF. The reading is typically between 300 and 600 ohms. It is adjusted for correct idle mixture CO on non- cat cars with the hex head adjuster which is on the side of the MAF, next to the 6 way connector. Note that this idle mixture pot is only in circuit with 84-86 cars. The early non-cat cars (S4, SE & GT) have a "remote"pot, which is on a bracket near the fuse panel. Cat equipped cars do not have an idle CO pot, as idle mixture is automatically set by the lambda loop.
4.To check the MAF has an output voltage: -. The easiest way to do this is to measure the MAF output voltage at pin 7 of the LH computer, which is in the side of the passenger footwell Bridge contacts 30 and 87 on relay XXV (relay XVI on '84 Euros) with a suitable jumper lead and measure for battery voltage between pin 2 (positive) and pin 4 (negative) on the disconnected plug at the MAF as a first check. The ignition does not need to be on. Reconnect the MAF, and measure the voltage at pin 7 on the disconnected plug of the LH computer (+ve) with the -ve lead of the voltmeter contacting a chassis or battery -ve point. The voltage should be about 1.7 volts d.c .If you get a helper to blow onto the hot wire, the voltage should rise up to a maximum of 5 volts. Note that this is only a crude check of circuit functionality, the MAF can still be out of calibration even if it passes this basic check. Zero voltage under all conditions indicates gross MAF failure - the car should have reverted to limp home mode.
5.To check the hot wire burn off: the car must be at normal operating temperature. Take off the upper air cleaner casing and remove the air filter. Start the engine, and take the revs above 2000rpm for a few seconds. Then switch off the ignition and observe the hot wire through the air filter housing. 4 seconds after ignition switch off, the hot wire should glow an red/orange colour for about 1 second."

svo-ed

TV and John,
Thanks for the continued help and advice with my problem.

TV: The car was not running when I purchased it. I think it had been sitting in a garage for at least a year when purchased. I suspect whatever is giving me issues was also providing a head ache to the previous owner. The car was purchased at an estate sale, so the previous owner was not available to speak to.

John: Your information is awesome. Thanks for going above and beyond. I will begin the process of checking as you have described. It may take a week or two, but I will work my way through the tests. (3 young children , a job and a demanding wife don't leave an excess amount of free time).

Thanks you to everybody for stepping up an helping me out with this!

Ed

auzivision

This is rather peculiar for sure and you already have input from folks much more knowledgeable than me. From a layman's perspective, I used to think of the go pedal as a gas pedal... when in reality it's more like an air peddle.

The throttle regulates how much air is fed into the engine, then the MAF measure the air and the LH calculates the amount of fuel delivered via the injectors. Sure, the LH considers a few other parameters like TPS position, temp, O2, ect.; however, in general more air equals more fuel resulting in more RPMs.

Now let’s look at your case. It appears at least at first more air equals more fuel... then something happens and the fuel delivery diminishes. When you apply more air, the engine coughs because it is being leaned out too far as opposed to pop misfire where fuel is present, but not in sync with proper spark. This is why I would suggest concentrating on fuel delivery as opposed to spark.

First, I would hook up a fuel pressure test gauge to the fuel rail (free loaner tool at Autozone) and observe the fuel pressure to see if it remains constant. If not, I'd chase mechanical fuel components like pumps, regulator, dampers, strainer, lines, etc. If it is constant, I’d chase electronic issues.

The fact that it fires at all means the pump is working at least initially and if I'm not mistake the electronics only turn the pump on and off. So it appears the pump is getting the signal to turn on... for the most part, once on it should remain on until the ignition is switched off. Sometimes relays have been known to misbehave, so a Quick jump of the relay and you’ll know if that’s it. Also, sometimes pumps work intermittently… I believe they have a thermal overload protection system that turns them off.

If the fuel pressure is constant, then it’s most likely the Electronic side like MAF, LH, Injectors, etc. It goes without saying that the MAF needs rebuilt as part of regular maintenance and John Speake is the resident expert on this and has already chimed in on how to diagnose these.

LH’s are also know to go bad (as one did this year in Hell and exhibited similar behavior). The only way I know to diagnose is to swap for a known good one. If you find a buddy that has a compatible LH and it works… then you know you need to get your LH rebuilt (or swapped for a rebuilt)… John has several vendors in the US that stock these.

My hunch, purely speculative, is it’s probably the MAF or LH. Many of the other items it appears you have already addressed… except for Relays, Relays, Relays. I believe they are typically mentioned in three because one each for the LH, EZK, and fuel pump... as cheap as they are, I’d replace all three and keep the old ones for spares or testing.

tv

I agree with this. In fact I just got up to post about an issue I had once on a group ride. My front damper went and it was causing trouble and if you pulled off the vac line there was fuel in it.

It was replaced of course and I then used clear vac line there to be able to see it in future. So check all 3 of those things.

auzivision

Did it ever run correctly after you replaced those three? IIRC a damper and a regulator look nearly identical, is it possible they are hooked up incorrectly? Say a damper is plumbed where a regulator goes... thinking out loud that the fuel pressure would fluctuate. I don't even know if that is possible, but worth a look.

If fuel is being sucked through the vacuum, it would be receiving more fuel than required and the RPM should increase with the throttle open... not drop... unless crazy rich to point of drowning engine. I'd presume the tail pipe would look like a diesel before that occurred.

Throttle open, initial RPM gain then loss still sounds like fuel starvation to me.

Melo928

How does car run if you actually drive it? Hesitation on accelleration, keeps speed easily steady or fluctuates? Go to limp home and only runs at 2000 rpms?

Even an airleak can effect on things when giving gas at neutral, driving will give you more information and hence easier to track down the problem. Airleak + MAF would be top in my list.