M. Requin

After a top end refresh, my 86.5, which had always run smoothly (it deserved the description, "purrs" and other 928-ers remarked how smoothly it ran) now runs like utter crap, to put it bluntly. Engine starts quickly and maintains an idle with seeking or surging, but runs as if misfiring on 1 or 2 cylinders, at idle and above. Engine has a distinctively even “lumpy” sound, and this seems to come from PS side.
Engine oil pressure 5 bar off idle, alternator output normal, coolant temp normal. No smoke from exhaust until it warms up, then just a trace, but exhaust pipe tips look damp after running.

Here's what I have done since January (not including hours and hours here looking for – and finding - suggestions). The first part of the project was the cam belt and sprocket replacement.
1. Installed new cam sprockets (aftermarket from 928sRus) and timing belt (Gates), timed with 32V'r (thanks, Seth!). Double checked timing and tension after a short run-in (1 hour) just to be sure it was correct
2. new rotors and distributor caps a couple of years ago, 1 new coil, 1 old coil
3. battery <1 year old

The car was not run before I then began the top end refresh:
1. cleaned and refinished (chemically stripped and painted) intake pieces
2. installed new gaskets
3. replaced all vacuum lines and breather hoses
4. removed air pump (did not remove any of the air pump plumbing)
5. new plugs (Bosch WR7DC+)
6. New Beru plug wires
7. injectors refurbished by Witchhunter
8. MAF refurbished by Injection Laboratories
9. cleaned and verified operation of ICV
10. Fuel pump pressure (static, bridged relay)) 34PSI (2.34 bar) @70°
Engine at idle 28 psi (1.93 bar)
11. verified that fuel dampers and fuel pressure regulator hold vacuum
12. Engine vacuum at idle 17" Hg, drops slightly when revved to 3K
13. Air box temp sensor 2.23K @ 80°
14. Checked plug lead resistances cap-to-cap all nominal
15. Inspected CPS output on oscilloscope, trace nominal, 2.5V peak-to-peak with clear timing pulses, no jitter or dropouts – looks just like the WSM, verified connection to EZF
16. TPS - verified operation of switches and continuity to LH connector
17. Compression, engine at ambient T:
67°F
1 = 178
2 = 175
3 = 178
4 = 183

72°F (later in the morning)
5 = 187
6 = 180
7 = 175
8 = 175
18. Plug readings, latest in pic
19. Verified all plugs fire with timing light
20. Disconnected O2 sensor, no change
21. Disconnected Temp 2 sensor, no start (is this right??)- resistance of both sides of sensor within range per WSM
22. MAF plug to LH connector - all connections show good continuity and no shorts
23. new Mahle air filter

Not done:

Haven't taken care of vacuum leaks associated with air pump removal
Haven't put the o'scope on the O2 sensor
Haven't looked at injector pulse width with o'scope
Haven't – to my satisfaction – ruled out bad ICV. Had been replaced in the past (pecker tracks all around, no wear as described by MrMerlin on armature)

I know I should have had these questions answered before I started this thread, but I have hit the wall so to speak, and need some input from other minds before I continue.

I'm gonna owe after this, but that's not a problem! TIA, fellow Rennlisters-

giddyupp

Definitely not an expert - but seems to me that you really need to take care of the Air pump vacuum leaks before you can even start to look for any other issues.

I am just about to wrap up my top end refresh - hoping to start her up on Saturday.

FredR

Marty,

Your vacuum at idle looks suspiciously low to me given my motor pulls 20 inches at idle. This being the case might indicate leaks leading to 1 or more cylinders running lean.

Difficult to read plugs from photos but I get the impression yours look different cylinder to cylinder which might give you clues where to look but perhaps those more expert in such can help you some more in that regard.

Compression pressures look sound- quite even so nothing to cause concern in that department.

Lumpiness is also consistent with vac leaks. I am not familiar with the nuances of fitting your inlet manifold but given the many pieces of the assembly it must be laced with sealing joints so perhaps you need to be looking at the integrity of such upon assembly.

A faulty coil can also do strange things but they tend to be such that breakdown occurs as the load/revs increases.

I do not know the fuel pressure spec for your model but presumably it is lower than the S4 given it runs 24 lb injectors and doubtless your would have found such to be faulty.

It will be interesting to see what others make of your syndrome

Good luck

Fred

PorKen

Double check plug wire order.

Mrmerlin

first thing to verify is what Ken Said/

Then these suggestions
did you recalibrate the MAF ? this is done by disabling the idle circuit? and setting the idle
NOTE this should only be done after any and all vacuum leaks have been sealed.

did you replace the short harness for the TPS? they go bad, they fail where they are connected to the holder on the water bridge.
did you replace the CPS? I dont mean checking it , NOTE this is a replace item when doing an intake refresh. the connector housing cracks.

Same goes for the ICV replace it as well the hoses that go to and from it.

Bill Ball

Triple check this. More than once I have found 6 switched for 7 or 2 switched with 3, even after double checking. These are long wires snaking over to the other side of the motor and right next to each other on the cap. Also, you need to verify spark to all plugs. You may have a bad or poorly connected plug wire or two.

Your idle vacuum doesn't look that off to me, especially if you are some distance above sea level.

soontobered84

For some reason, 6 and 7 have bitten me a couple of times. Good thing is: It's easy for me to find now.

GT6ixer

Longshot and very rare but it's possible that you got a bad spark plug out of the box. As a check replace them with the ones that were in there when it was running well.

dr bob

Martin,

Expanding some beyond the great advice so far,

-- Look for Dwayne's write-up on intake refresh. Look particularly for the drain-pipe-fittings piece he made to let you put air in the intake to pressure-check. Put the engine at the 45º mark, add no more than a few PSI of air (think "regulator"), and listen for air leaks.

-- Your o-scope can be adapted easily to monitor relative firing voltages with a couple DIY "inductive pick-ups". Just take a few short (~4") pieces of solid single-conductor wire, wrap one around the coil wire and one around the #1 plug wire. Wind the two wires in the same direction but opposite spiral. ie: coil wire clockwise with loose end towards the coil. #1 plug gets the clockwise with loose end towards the cap. Read the coil wire by connecting your scope lead to the wire, and use the #1 plug wire the for sync. With the engine running, adjust the 'scope so you can see all eight cylinder spikes at the same time on the screen. Each cylinder will appear in firing-order from the distributor end, starting at sync #1. In a perfect-running car, the spikes will be the same height. Deviations are caused by plug problems, cylinder pressure problems, wire problems. Actual firing voltage will vary due to differences in plug gap, crud on the insulator, and also from gaps/resistance in a plug wire, or a fault to ground in a plug wire. #1 wire is not very long so it's easy to verify that it's physically OK and suitable for using as the sync source.

-- Check the individual injectors with a stethoscope with the engine running. A loose connector or broken conductor will keep the injector from clicking, easy to find with a stethoscope really.

-- Don't be afraid to use masking-tape "flags" on plug wires at each end, confirmed with the ohm meter if necessary. The factory wires came with cylinder numbers on each cable, a function you can duplicate with temporary tape-tags. Or more permanent with a p-touch label if needed.



Are those compression readings taken -after- the work was done?

mark kibort

does it run smooth at higher RPM? under load, or full load (WOT).

if it does, the leaks would not be the issue. it should be smooth as silk under load even with leaks. idle should be the only issue here if i understand the observation so far. if its rough part throttle and higher RPM, double check spark on all plugs and of course match rough running with injectors for those cylinders too.. do a compression test. MAF out of wack wouldnt do it either. our friend with the 6 liter had a maf 5 times out of spec... it wasnt the problem..... something else, but did the calibration and tune on it and i think it runs a little better now. not a huge deal though. you got something else going on here.

Snark Shark

Did you tighten the idle screw on the throttle body to make it idle at the correct RPM in spite of the vacuum leaks? That could make it stumble and miss at idle as the MAF is reading almost no air. Might need to let it idle high if you must run with vacuum leaks.

M. Requin

Great ideas, gents- thanks! I'm doing my interpreter thing the next couple of days so it will be Sunday before I get back to it, and then I'll reply to the specific points you have raised. One thing that has occurred to me is to double-check the PS intake manifold gaskets. I have remembered that I had a difficult time getting one to stay in position, and it may have slipped- this could account for the large air leak and the localized chuffing sound, maybe. Unfortunately, "what is the last thing you touched?" is good advice, just hard to follow when you have touched almost everything...

Adk46

For what it's worth, the closest I came to ruin on my intake refresh was barely catching a wayward cam-cover gasket. It could have happened on the intake manifold.

My actual faults included a disconnected vacuum line near the brake booster, and something amiss on the throttle quadrant. Finally, I got a tip that I should not expect smooth running without the airbox/filter/intake pipes installed, or the hood off.

Hats off to anyone who does this complex job without any faults. You'll find yours, Martin.

928S MN

Hi Martin,

In the venue of improving the knowledge base of the 85-86.5 model years, what was the eventual outcome of your troubleshooting?

M. Requin

I know I haven't followed up on this thread, but I have done quite a bit of work. I'm on a tight schedule this morning, but will try to get it together and post where I am on the project later today/tonight. Thanks for getting me off the dime.