Advice - 924s Hunting RPM at Idle

[andyman_2006]

Hey Porsche Classic people.

I have a 1987 924S, 84K miles, and it was running fine last year, hasn't done lots of miles in last 2 years, but has now developed an unusual (to me) fault. Car starts ok, however does smell rich though, idles ok until starts to get warm, then once warmer tick over starts to hunt between 900-1200RPM if i rev gently up to 1500RPM its steady idle hunting goes away.

I've changed the following recent:

-Plugs
-Leads
-Dizzy and Cap
-Air filter (was part of service anyway)
-Oil and filter (part of service)
-It's Had Belts, tensioners and water pump done last year
-New Idle air valve (fitted this weekend)

Only thing not changed so far is AFM, considering a refurbished one from Frazerpart.

Starting to Lose faith in it now :-( Car drives great apart from the Idle.

Any help or ideas would be welcome???

[cd1957]

Hi there are temperature sensors to consider, also fuel pressure and idle stabilizer can go funny.

Have you actually check mixture with a gas analyzer?

Chris

[andyman_2006]

cd1957 said:

Hi there are temperature sensors to consider, also fuel pressure and idle stabilizer can go funny.

Have you actually check mixture with a gas analyzer?

Chris

Hi Chris,

Thanks for reply, no not had the car on a gas analyser yet.

So temp sensors could cause hunting idle? my temp seems very stable, does not jump or read high/low, seems consistent with car when cold, then when warming up, but i'll order one and swap it, presume this is the one behind the air box/near the AFM? is there another you mention sensors?

Fuel pressure regulators look to be old and original, has a red seal on.

Its the Idle valve (ICV or ISV as some seem to refer to it as) that i have replaced the one hard to get to under the inlet manifold.

Regards
Andy

[cd1957]

Hi Andy,yes stabilizer under manifold.
The temp sensor you need is the one with the Blue plug connector,this is dme one.

The other is temp sensor 1,it's in the airflow meter,and specific resistance values through the temperature range.

Chris

[andyman_2006]

Thanks Chris,

So I've changed that valve under manifold. But it's not resolved it, it now revs up slowly over about 1 min to 2k rpm, sits until nearly warm, then drops slowly to 1k then starts hunting at idle again:-(

I've Ordered the temp sensor (blue connector one) today, is the one in the air flow meter changeable or is it a part of the closed part of the AFM?

Would you consider a new/refurb AFM a good call? At £222? Or should I be looking elsewhere?

Someone has suggested possible ECU faults :-( I hope not.

Regards
Andy

[cd1957]

Hi Andy, I may have dme testplan, which may help guiding you through the system.

Will see what i can find.

Chris

[Huskyman]

I know this sounds pessimistic but have you had a sniff of the coolant expansion tank, or carried out a compression leak down check? What condition are the plugs in? I remember being told that an unstable idle can be caused by this. Any excess vapour from the exhaust?

Good luck with this.

[andyman_2006]

cd1957 said:

Hi Andy, I may have dme testplan, which may help guiding you through the system.

Will see what i can find.

Chris

Thanks Chris, appreciated.
Andy

[andyman_2006]

Huskyman said:

I know this sounds pessimistic but have you had a sniff of the coolant expansion tank, or carried out a compression leak down check? What condition are the plugs in? I remember being told that an unstable idle can be caused by this. Any excess vapour from the exhaust?

Good luck with this.

Hi, thanks for the reply.

No not had a sniff of exp tank, to be honest never really crossed my mind could be looking at HG, so not done Leak down or comp test either, car was running ok last summer, been on SORN for Winter. No mush in the oil filler cap though i do recall looking at that - but thats over a year ago before i bought it - so will check cap, and sniff exp, i have a comp tester so can do that, ideally need an air line for Leak test i think?

Spark Plugs (before i changed them were Black - quite sooty so pointing to Rich running?)

No excess smoke, white or other, slight puff of blue on first cold startup, clears after about 30-60 secs. Tailpipe quite sooty though - again pointing to rich running i suppose?

Frustratingly cars drives well, above idle rpm that is!

Andy

[LarJammer]

Your symptoms sound like a faulty throttle switch, i believe there is a guide on clarks garage for testing and adjusting it.

[andyman_2006]

LarJammer said:

Your symptoms sound like a faulty throttle switch, i believe there is a guide on clarks garage for testing and adjusting it.

Thanks for this, I've looked into it, and found the Clarks article.

Slightly miffed at what Porsche seem to think is acceptable prices for some of these parts, i mean £200 for a switch? and nearly £1000 for an AFM...and then £49 for a coolant temp sensor, which was £7 for my E-type...

Will test the TPS and see if its faulty.

Cheers
Andy

[Huskyman]

andyman_2006 said:

Huskyman said:

I know this sounds pessimistic but have you had a sniff of the coolant expansion tank, or carried out a compression leak down check? What condition are the plugs in? I remember being told that an unstable idle can be caused by this. Any excess vapour from the exhaust?

Good luck with this.

Hi, thanks for the reply.

No not had a sniff of exp tank, to be honest never really crossed my mind could be looking at HG, so not done Leak down or comp test either, car was running ok last summer, been on SORN for Winter. No mush in the oil filler cap though i do recall looking at that - but thats over a year ago before i bought it - so will check cap, and sniff exp, i have a comp tester so can do that, ideally need an air line for Leak test i think?

Spark Plugs (before i changed them were Black - quite sooty so pointing to Rich running?)

No excess smoke, white or other, slight puff of blue on first cold startup, clears after about 30-60 secs. Tailpipe quite sooty though - again pointing to rich running i suppose?

Frustratingly cars drives well, above idle rpm that is!

Andy

That probably rules out the hg if the plugs are not steam cleaned then. I would see if the idle switch can be adjusted before condemning it. You probably have access to a multimeter to test it and set it up. If you suspect the AFM then have a look here https://www.frazerpart.com/acatalog/Porsche-Airflo...

A compression test and leak down test can give a good picture of the overall health of the engine and can help with diagnosis.
Good luck with this and keep us updated on how you get on

[andyman_2006]

Huskyman said:

andyman_2006 said:

Huskyman said:

I know this sounds pessimistic but have you had a sniff of the coolant expansion tank, or carried out a compression leak down check? What condition are the plugs in? I remember being told that an unstable idle can be caused by this. Any excess vapour from the exhaust?

Good luck with this.

Hi, thanks for the reply.

No not had a sniff of exp tank, to be honest never really crossed my mind could be looking at HG, so not done Leak down or comp test either, car was running ok last summer, been on SORN for Winter. No mush in the oil filler cap though i do recall looking at that - but thats over a year ago before i bought it - so will check cap, and sniff exp, i have a comp tester so can do that, ideally need an air line for Leak test i think?

Spark Plugs (before i changed them were Black - quite sooty so pointing to Rich running?)

No excess smoke, white or other, slight puff of blue on first cold startup, clears after about 30-60 secs. Tailpipe quite sooty though - again pointing to rich running i suppose?

Frustratingly cars drives well, above idle rpm that is!

Andy

That probably rules out the hg if the plugs are not steam cleaned then. I would see if the idle switch can be adjusted before condemning it. You probably have access to a multimeter to test it and set it up. If you suspect the AFM then have a look here https://www.frazerpart.com/acatalog/Porsche-Airflo...

A compression test and leak down test can give a good picture of the overall health of the engine and can help with diagnosis.
Good luck with this and keep us updated on how you get on

Well hopefully thats some good news then, i have spoken with Max @ Frazerparts, and he has offered to test/refurbish my AFM, he can also test and rebuild the DME ECU, but i'm hoping this is not needed.

Yes have a multi meter and will take a look this weekend, will fit the new coolant temp sensor as well.

Thanks for the posts.

Andy

[cd1957]

Hi Andyman, sorry for delay.

I have a dme testplan, I would try scoping duty cycle on the stabiliser and injectors, can you unplug stabiliser.

I would also look at what the mixture is actually doing.


Chris

[andyman_2006]

cd1957 said:

Hi Andyman, sorry for delay.

I have a dme testplan, I would try scoping duty cycle on the stabiliser and injectors, can you unplug stabiliser.

I would also look at what the mixture is actually doing.


Chris

Hi,

You dont know where i can obtain a factory PDF for the workshop manuals do you? I've found the non 's' 924 one, but struggling to find the 924s one.

Update: Took off the air box, AFM, and hose to the throttle body, and noticed a large amount of sludge in the large hose, and the small one that feeds into the main hose, and entry to throttle body housing/butterfly. Cleaned it out, but was surprised to see it, looked like a mix of oil/carbon build up. also found a small vacuum hose off towards back of the inlet manifold, refitted but made no difference. Does anyone know if the sludge is normal? or where it may have come from?

Ordered new TPS - which is here now, and fitting that tonight, along with the coolant temp sensor.

On the scope of Duty and injectors - i dont have a scope, so would need to consult a dealer/garage or someone in the trade who has one. :-(

I have replaced the stabiliser valve - for a new one, the one under the inlet with the hoses on each end. The old one actually looked ok, and was clean inside and replacing this has made no difference to the rough hunting idle.Since the new ICV/stabiliser was replaced the car now revs up to 2k rpm from cold, once warm drops to around 1k rpm then hunts between 800-1100 :-( suggestion was this is symptomatic of TPS switch fault and testing the current one with a multi meter showed area for concern. so lets see tonight - fingers crossed!

Will also do a compression test tonight see if that throws any alarming results.

So unconfirmed good/bad parts are DME/ECU and AFM - which i can send both away to Fraser parts and have them tested/refurbished. Hoping this is a last resort though.

Its starting to annoy me now,I've been patient, but i may have to just admit defeat book it in with a specialist and see where it ends up - though i'm reluctant to blank cheque book it...

Andy

[andyman_2006]

Ok, so TPS and Coolant temp sensor changed.

Its till the same :-(

The Reving up to 2k rpm has gone, it just idles at 800-1200 hunting (same as before) even from cold.

Considering replacing the AFM for a fully refurbished item, i did consider a used one but they are £75 on ebay, and likely old and worn so no better than my 30 year old one fitted, so will see if this resolves, and then i'm done, i give up :-(

Andy

[R36vw]

Watch the latest wheeler dealers(last week)
Porsche 924 with same issue turned out to be the fuel pressure regulator. Worth watching and seeing if thats the route you need to take. Very little cost if it is.

[andyman_2006]

R36vw said:

Watch the latest wheeler dealers(last week)
Porsche 924 with same issue turned out to be the fuel pressure regulator. Worth watching and seeing if thats the route you need to take. Very little cost if it is.

I've been doing more tests, and it seems could well be the FPR or the FPD maybe both (They are £251+vat each) so £602.40 for both parts not exactly cheap!....i'm amazed at what porsche charge for some parts really.

Replacing the AFM didn't work - but at least i have a newly refurbished one (they are well known to fail with age)

[R36vw]

Ouch! Thats not a cheap repair. The Fpr could be worth stripping down and giving a really good clean. You've nothing to lose. Good luck

[andyman_2006]

R36vw said:

Ouch! Thats not a cheap repair. The Fpr could be worth stripping down and giving a really good clean. You've nothing to lose. Good luck

Long time since this post for help - but in summary sorted it last month, after a busy summer using other cars, and losing a bit of interest in the 924s.

Anyway, Fuel pressure regulator, and fuel pressure damper both changed, and sorted the hunting tick over issue - and then it presented itself a 2100rpm idle! which turned out to be a duff pattern part ICV i fitted (lesson learnt!) re-fitted the old Bosch ICV and its now sorted.

Final fix was a new oil pressure sender which was weeping oil, and so its back in tip top.

Car SORN for winter, so will get it used and some miles next March.

Thanks all for tips and advice.
Andy