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'84 s2 missing and stumbling

Gentlemen, I am nearing the end of my rope here. I am an ex dealer mechanic currently working on an '84 928 s2, euro model, no cat. While I have loads of aircooled experience these things (928's) have always been pretty thin on the ground at the dealer (no surprise given what they charge) and as such I have only worked on a handful and I am running out of ideas as to what could be the trouble with this one.

So, I come to the brain trust in hopes one of you can point out something I may have missed.

As stated the car is a 1984 928S2, euro model, no cat. It runs great with a slight surge on full throttle when cold, and once it warms up it picks up a strong missfire / stumble that eventually makes it undrivable. This happens pretty mucvh as soon as the temp gauge reaches normal. Idles pretty good if slightly lumpy, ok at part throttle (but not great) and at WOT it really stumbles and surges. Feels like it is running very lean. Revs to about 4k RPM under these conditions then struggles to rev any higher. And as it warms up to fully heat soaked this gets worse and worse such that eventually it becomes dangerous to drive in traffic as it cannot really get out of the way at intersections.

Compression is even at around 160 on all 8
plugs are new (ish) and white indicating lean running
cap, rotor, coils, all new.
plug wires check out as ok
fuel pump new, fuel pressure at test port is 25 psi at tick-over/idle
both FPR's are 6 months old.
I've set the CO to 1.5 with a CO meter, lambda (at idle of course) is 1.04 (seems reasonable)
unplugging the MAF seems to indicate the MAF is good ie: it then goes into limp mode, so I know it is not now in limp mode. I have also checked the resistance of the MAF and this checks out relating to info from JDS Porsche. I cannot convince myself it is the MAF.
Outputs from both Temp sensors in the coolant bridge seem to be reasonable at both hot and cold.
Found the massive rubber union under the center intake at the T-body split. Replaced this and now I also have a high idle.
I've checked all visible ground points at the engine and they all look good.

Seems to me that during the enriched warmup phase it drives fine, as soon as it is fully warmed and pulls fuel back to normal running it becomes very lean.

Throw ideas at me, I am running out of them.

Customer is a good guy and before I start replacing fuel injectors, ecu's and MAF's I want to make sure there is nothing else I could have missed.

Any ideas?

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Old 09-03-2013, 07:18 AM
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Are you sure you got all the air/vac leaks sorted? An 84 needs all new rubber. A full intake refresh is in order for this car.

Also, not sure in the UK, but you can get those Injectors rebuilt and flow matched/tested to verify they are fine. I do know I had a Mitsubishi Eclipse Turbo that the injectors flowed perfectly, but once it got to temp, it started to miss really bad. And this was after all the injectors were tested.

Does this model have a Green Wire? If so, what color is it now?

Post pics of the car and especially the eingine compartment.
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Old 09-03-2013, 08:46 AM
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Nope this is a dual dizzy engine so no green wire to worry about.

I have had the intake plenum out and closely inspected all 8 of the runner boots. They all look good. As I said I replaced the big rubber that joins the plenum to the throttle body. And I have been over all the other vac hoses closely. While I can find no leaks, I can hear a distinct hiss at idle with the airbox cover off. I presume this is normal and I am just hearing intake hiss thru the throttle body. With the airbox back together it is only just noticeable.

Customer tells me the injectors were cleaned in the past, but I am beginning to wonder how recently or how well this was done. Thinking at this stage I may send the MAF off to be tested (JDS does this cheap) and if that comes back ok, the injectors become suspect.

Just wanting to catch all the "gotcha's" before we make an investment in injectors so checking in with the experts as it were.
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Old 09-03-2013, 01:01 PM
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Have you pulled the injectcors and had them cleaned? Many rough running issues are caused by dirty injectors.
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Old 09-03-2013, 02:02 PM
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Check the Temp 2 sensor
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Old 09-03-2013, 08:39 PM
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New cps is probably in order, preemptive.

And, clean the two important ground wires located on the passenger cam cover. This is a simple step; its where the engine electronics all ground as you should remember on S2. CPS and grounds / CE cleaning solved missing for our S2.

Another possibility is an intermittent contact fray at an injector or a fray at MAF plug.

Possible dirty ground, fuel pump, behind right rear quarter.


I find every 2 years needing to pull the CE panel and polish the fuse holders, and re-fuse. Relay replacement for the critical ones might be in order, too. LH, EZ and Fuel pump relays.

Ground Cleaning, 16V Euro

Separate issue, Cutomer car --- have the fuel lines been replaced?
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Last edited by Landseer; 09-04-2013 at 04:34 AM..
Old 09-04-2013, 04:07 AM
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Quote:
Originally Posted by stepson View Post
Check the Temp 2 sensor
S2 Euro behaves a lot like US 85/86, so I was thinking the Temp 2 sensor.

Whats the tail pipe smell like, gas?

How do the plugs look?
Old 09-04-2013, 08:26 PM
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after checking the grounds as Chris points out,
NOTE there may also be some at the back of the V,

I would do a firefly test,
remove the intake tubes and run the car at night look for jumping sparks,
if you see any then a new set of wires is called for , and it probably wouldnt hurt to add them to the engine now

Make sure the coil wires are not touching any metal parts as they will rub and short out.

Get the MAF rebuilt, this will do wonders for drive ability.

Get the injectors cleaned.

Check the timing of the twin dizzy replace the belt if necessary.

check the timing of the cams / crank/ dizzy

replace the temp 2 sensor and possibly the Bosch plug that connects to it.

clean the grounds at the chassis and behind the right Q panel in the hatch floor.

for the high idle it may need to be turned down after correcting the vacuum leaks.

I know I have added a lot of things to do here,
BUT if you dont want to see this car again,
then all of the pieces parts must be in top condition for continued performance,,
so often only one or two of these parts gets repaired and the engine still runs but not well,
now its running like shmack so you have to restore all of the critical components
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Last edited by Mrmerlin; 09-05-2013 at 10:43 AM..
Old 09-04-2013, 08:42 PM
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+1 to what Stan posts above
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Old 09-05-2013, 10:14 AM
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how you coming along?
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Old 09-07-2013, 08:32 PM
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Right, the plot thickens. Spoke to JDS Porsche and he gave me typical vac figures at the fuel dampner. He says 24 inches of mercury, I have 22. Close enough. I have taken off the plenum, throttle body, and the trottle body to MAF tube and thourougly cleaned all of it. T-body was filthy and no doubt not helping any of this. Very closely inspected all the vac hoses and fittings and replaced a few iffy parts, but nothing that was screaming out as being the problem. Also fitted a new crank posit sensor as it was cheap enough and seems to be a common fault. cleaned all the grounds in the engine bay, the two on the left rocker cover and one down the front of the engine on the right (of the car, left when viewing). I've confirmed with the customer that the injectors were recently rebiult/cleaned. about a year ago. sent the MAF off to JDS to be checked, he said it was ok, he recalibrated it (for free no less, damn good people over there) so I know its ok. I have checked the condition of the cog belt in the dizzy, looks fine, very little wear at all. I've been over the engine wiring harness with a fine tooth comb paying particular attention the the injector wiring, and while it has a few past repairs, nothing that looks like it could be a fault. Where it has been fixed in the past (2 places) I have tested it for contunuity while wiggling it and its all ok.

So took it for a test drive again today having just put it all back together and the problem persists. However, there is a new clue. I have found that the speedometer is reacting to the throttle. At a steady very light throttle cruise the speedo works as it should, as soon as any attemt to accelerate is made, and I mean just a bit, or floored, the speedo drops off by 10-15 percent and twitches around down there, release the throttle back to cruise, and it snaps right back to where it should be and stays there, nice and steady. This indicates to me thqat all of this is going to come down to an elec problem either in the CE board or the ECU itself. JDS is willing to test the ecu for a nominal fee so I am going to mail it up to him purely to eliminate it as a possible problem, and then sink my teeth into the CE board. The customer tells me that he has been thru all the fuse contacts and relay plugs with a dremel prior to bringing the car in to me, and I trust him to have done a good job, however he also tells me that this thing is a bit of a mess in there as it has had various phone kits, alarms, and a towbar (!?!) thru the years.

So thats where we stand and thats how I am going to attack it. Ill post here when I finally find the fault so the next guy may get an answer a bit quicker, and as always if any of you have seen this before feel free to chime in with advice.
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Last edited by Blaize; 09-20-2013 at 03:10 AM..
Old 09-20-2013, 03:08 AM
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I think there is a bad connection in the CE panel,
and possibly has been damaged by CLEANING with the DREMEL.

your computers are probably OK,
BUT if you can get them checked why not ,

inspect the computer connectors while they are off,
its common to find corrosion.

Look at the CE panel along the top if you see rust then the blower box is leaking,
so I would suggest that you verify every fuse is correctly placed and also has the right feel as sometimes the fuses dont get clamped well.

Do the same for the relays, make sure they are clean, any relay that has corroded pins should be opened.
Verify correct PN and position
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Old 09-20-2013, 06:12 AM
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UPDATE... In testing and checking in and around the CE panel, I found a corroded pin on the DME plug. Pulled and opened the DME unit and found it has been wet at some point in its life. There is a diagonal "tide line" across the board inside and as you can imagine a fair bit of corrosion on various components. Nothing that screams out to be the problem, but lots of crusty bits. I have sent this and the ignition control unit out for testing as a matter of course, but I have a strong feeling his is going to be what was wrong with it. The DME had paint pen numbers written on it so I suspect it has been replaced once already wit a unit that perhaps sat around the junk yard for a little too long. I doubt it is cost effective to repair it, so I will be hoping we can find a better used one this time around.

watch this space, I'll update once I have more info.
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Old 10-03-2013, 03:48 AM
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excellent, thanks for checking-back, been wondering about this one.
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Old 10-03-2013, 03:58 PM
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What kind of shape is the battery in?
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Old 10-04-2013, 10:34 AM
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The DME should be sent to John Speakes in UK.

He is the master at the 928 LH, EZK and MAF rebuilds.
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Old 10-04-2013, 04:02 PM
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Well they are back in and it didn't fix it. The battery seems ok to me, starts fine every time. I have not done a test on it, though it doesn't take any time so I will.

John Speake has now rebuilt the MAF, tested the EZK and replaced the LH. In fact he told me he drove his own car around with the LH and EZK from me fitted just so he knew they were ok.

yesterday I set up a fuel pressure test rig so that I could drive the car and watch the fuel pressure as I drove around. Having done that I have 25 PSI at idle, 26 PSI at cruise, and 34 PSI at WOT. always pretty steady. that's 2-3 PSI below spec on all counts, but not enough to be causing the problem here I don't think. (and there is every chance my gauge is a bit off as it hasn't been calibrated in ages). So that satisfies me that the fuel pump, filter, and regulators/dampener are all ok.

I suppose the next step is to pull the injectors and have them tested and cleaned. I pulled all the plugs yesterday to see if there was any variation in color (and perhaps point us to a bad injector or two) and found them all even and pretty lean.

Going to call John Speake in a few min and bend his ear for ideas, I am about ready to throw a match into the tank and be done with it......
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Old 10-09-2013, 02:44 AM
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You might be wise to replace the bat ground strap. They are notorious for corrosion beneath the plastic cover. Especially if moisture has gotten into the car, which it seemingly has. Will produce some really oddball, flaky runability issues. Inexpensive. May test at 12V, but not pass enough current, kind of like sand through the neck on an hourglass, then changes character with vibration and as it warms and with varying load.

Injector cleaning also makes sense, but already done? Hmm.

Tide line on panel cannot be good.

Stan, not sure what you mean by CE / Dremel issue? Loosing wire strands in panel? Aren't CE leads brass, not plated?
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Last edited by Landseer; 10-09-2013 at 03:58 AM..
Old 10-09-2013, 03:53 AM
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from post 11
The customer tells me that he has been thru all the fuse contacts and relay plugs with a dremel prior to bringing the car in to me, and I trust him to have done a good job, however he also tells me that this thing is a bit of a mess in there as it has had various phone kits, alarms, and a towbar (!?!) thru the years.

I think there are issues with the CE panel drop it and look for wires with a sheen to them this indicates they got hot make sure none are melted into each other.
I also think the ignition wires are suspect the twin dizzy inspection also includes making sure thet rotors are both pointing at the hash marks at TDC, the little belt can look good but the timing can be off.

I would also go back to the CE panel and remove every relay and confirm its the correct poart number and its in the correct position.
There are more than few times when a PO has installed the wrong relay and or installed the relay in the wrong spot.

Go to the spare tire well and clean the grounds in the well,
and also look for a loom of brown wires going under the rear quarter panel liner,
and clean these.
A new set of ignition wires is needed
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Old 10-09-2013, 06:23 AM
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Thanks for clarifying, yes that all makes sense.

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Old 10-09-2013, 04:00 PM
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