stevedavies

928 S4 1991 Euro RHD Auto

Have had a high idle problem with my S4 for some time. At cold the idle is approx 800rpm, rising to 1100rpm when warm. I checked the TPS at the LH and EKZ ECU and was not open circuit under all conditions. Decided to do a complete intake refresh last month as per Sir Dwayne. Changed all rubber hoses, TPS, ISV, flappy valve, all vacuum lines, knock sensors, throttle bearings. Had injectors cleaned while I was at it. Put it all back together this week, checked TPS at LH and EKZ ECU, worked perfectly. Tried engine and idle exactly the same!! Not sure what else to try??? Changed all vacuum lines under hood?? Changed check valve on 4 way vac splitter under hood also. Could vacuum components inside car eg heater aux unit affect idle speed??

Any thughts would be appreciated

Tails

I would check the adjustments of all linkages to the throttle, cruise control and auto box and set them up as per the WSM. If you don't have a WSM buy a set of Jim M's Technical CD from Roger at 928sRus.com.

I would also check the Idle Stop Valve (ISV) is not sticking. Best treatment is to give it a good dose of WD40 down the suction tube just after the splitter adjacent to the throttle, crusie control and auto box linkage.

If this does not work then disconnect the battery negative terminal, so as to reset the computers as the idle speed is electronically controlled and when the battery is reconnected it should adapt and the idle should be reset to original specifications.

If this does not work I would see if you can borrow a DT928 Diagnostic Tool and do an adaption set up.

Tails 1990 928S4 Auto.

stevedavies

The 1100rpm idle speed is constant and I can hear it hunting around it, so assumed the new ISV is ok (this was the same behaviour with the old ISV). Checked throttle linkages and all seems ok. I can move the cable manually under the hood and hear the TPS clicking. Would vacuum leaks in HVAC cause idle problems?? It the only part of the vacuum system I have not checked.

John Speake

If it's hunting around then that is a good sign that the LH and ISV are OK.

Tommy928

I had the same problem and I went through everything as you had, twice, and still high idle. I even replaced the LH and EZK but no luck. I finally solved it by replacing the mass air sensor.

cpayne

I had the same thing. replaced the MAF problem gone

stevedavies

Update on high idle problem.. I checked vacuum at front fuel damper, reads approx 19 to 20in HG. I then attached connector to MAF ( as per Sir Dwayne) to enable me to pressurise the intake. Took it to approx 1.5 and it takes approx 20 secs to discharge. Is that good or bad??

Mrmerlin

I would suggest that you inspect the 2 plastic junctions on the sides of the MAF rubber boot they get hard and crumble it may not be evident that one has a crack. replace both sides dont overtighten the clamps

stevedavies

Forgot to add I changed all rubber hoses inc MAF rubber boot and plastic connectors. All seemed to be tight when I assembled it

stevedavies

I rechecked the intake pressure. It goes from approx 1.5 to 0 in about 15 seconds. Is that large ? |Could it be responsible for the high idle of 1100rpm??

the flyin' scotsman

Steve.............I have exactly the same issue. Refreshed intake with rebuilt MAF and LH. All vac hoses and other under intake hoses changed.

When the engine is stone cold it idles at approx the correct value but its stumbling and as it warms the idle increases and the stumbling ceases.

I previously deleted the air pump assembly including the diverter valve so I'm hopeful that the vac port in the throttle body is leaking. I'm taking the intake off again when winter arrives.

I should add that I recently removed the fuel rails and injectors looking for a bad Oring (I had my injectors cleaned also) but everything was good and the car runs the same as previous to the fuel rail/injector R&R.

Another point, with exception of the high idle the car runs strong.

The HVAC system can easily be omitted in your trouble shooting by plugging the output of the 4 way tee at the brake booster.

Did you change the bearings for the throttle shaft and flappy? I'll be doing those also.

stevedavies

Malcolm, yes I changed the bearings at the throttle shaft and flappy(from Roger Tyson).I have also plugged the 4 way vacuum connector to eliminate the aux units inside car.
Your description sounds exactly the same, idle approx ok at cold, but rising to 1100rpm whe warm. Speed is rock solid, and gently hunts around the 1100rpm. Car runs fine and is better since the refresh.Only issue seems the high idle speed.
I checked the TPS idle and wot at the LH and EKZ units and were both good. Am running out of ideas !!

the flyin' scotsman

I'm out of ideas also. My hope is I find something very evident when the intake comes off and will post findings.

I may also try another MAF if I can 'borrow' one from a good running car although my JDS rebuild is not that old.

Before I installed the PEMs in both brains I ran a full diagnostics check with Theos diag. system (you need OEM eproms) all was good.............if you have access to this or one of JDS tools you may want to try all the various sensor checks and idle adaptation.

Also, I sprayed brake cleaner fluid (NON FLAMMABLE) around the intake, engine running and it was very evident theres a leak as the fluid was sucked in under the intake and the resultant white vapour from the tailpipe.............try that but I stress NON FLAMMABLE.

stevedavies

Malcolm, I am resigned to taking off the intake again at weekend to have al ook. I have pressurised intake today and there is a leak. I dont know if its big enoungh to cause the high idle ( takes approx 15 to 20 sec to discharge to zero from 1.5 ).
The confusion for me is the idle speed/characteristics are exactly the same before and after intake refresh ( i knew the TPS was open circuit before refresh). Suggests it may be something else involved. Will let you know how I get on!

Mrmerlin

put in a new rebuilt MAF before you pull the intake