Newbie needs help - High idle
07-27-2006, 05:54 PM
#1
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Newbie needs help - High idle
Hi all,
I just bought my first 928 yesterday, a Euro 1985 S2. Drove it 300 miles home, and I'm really happy with it!
Had some time today to play with it today to try to troubleshoot a rough (and slightly high) idle, I noticed that the large vacuum hose between the left of the throttle body and the plenum was plugged at both ends... I connected them back how they should be, and my idle shot right up.
I tried adjusting the idle with the large screw on the front of the throttle body, but even when wound all of the way in it was still surging at around 1200.
Looking further into it, I tested the throttle position sensor to be good ie shorted when throttle closed. It made no difference to the idle or car running with this disconnected. The MAS was replaced just over 2 years ago, and still looks pristine. I also tried unplugging this, with no change. Under the plenum there is what I thought to be the Idle air valve (2 large pipes going into a rectangular slim unit with a connector to one side). Again, this was unplugged, with no change.
Upon doing a search on here, as far as I can tell, the Idle air valve is a conventional looking unit tucked right underneath so you cant get to it, so it probably wasnt that I unplugged.
I heard that some of the early euro LH Jet cars didnt use an Idle air valve, is this true?
Does anybody have any suggestions as to what I should do next?
Kind Regards,
Ben
I just bought my first 928 yesterday, a Euro 1985 S2. Drove it 300 miles home, and I'm really happy with it!
Had some time today to play with it today to try to troubleshoot a rough (and slightly high) idle, I noticed that the large vacuum hose between the left of the throttle body and the plenum was plugged at both ends... I connected them back how they should be, and my idle shot right up.
I tried adjusting the idle with the large screw on the front of the throttle body, but even when wound all of the way in it was still surging at around 1200.
Looking further into it, I tested the throttle position sensor to be good ie shorted when throttle closed. It made no difference to the idle or car running with this disconnected. The MAS was replaced just over 2 years ago, and still looks pristine. I also tried unplugging this, with no change. Under the plenum there is what I thought to be the Idle air valve (2 large pipes going into a rectangular slim unit with a connector to one side). Again, this was unplugged, with no change.
Upon doing a search on here, as far as I can tell, the Idle air valve is a conventional looking unit tucked right underneath so you cant get to it, so it probably wasnt that I unplugged.
I heard that some of the early euro LH Jet cars didnt use an Idle air valve, is this true?
Does anybody have any suggestions as to what I should do next?
Kind Regards,
Ben
07-27-2006, 06:32 PM
#2
Drifting
Pictures would help, especially with arrows on them. Often a high rough idle is the result of a vac leak somewhere. I once had one on my S2. Is this an auto or 5 speed. Maybe your vac lines are messed up, there are lots of them.
07-27-2006, 07:37 PM
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Its an auto transmission.
I'll take some tomorrow of the various parts. My initial thought was a vacuum leak, but would this cause such a high idle even with the adjustment set to minimum?
I think there must be some vacuum leak as the transmission shifts fairly hard compared to my Mercedes (with the same transmission). I take it the line to the tranny is the solid line on the plenum held on with a banjo bolt?
Ben
I'll take some tomorrow of the various parts. My initial thought was a vacuum leak, but would this cause such a high idle even with the adjustment set to minimum?
I think there must be some vacuum leak as the transmission shifts fairly hard compared to my Mercedes (with the same transmission). I take it the line to the tranny is the solid line on the plenum held on with a banjo bolt?
Ben
07-27-2006, 10:39 PM
#5
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Ben,
The idle adjustment screw is basically a controlled vacuum leak (sort of, it's measured air). If you have another, larger, uncontrolled vacuum leak, the idle screw won't do much.
The idle adjustment screw is basically a controlled vacuum leak (sort of, it's measured air). If you have another, larger, uncontrolled vacuum leak, the idle screw won't do much.
07-28-2006, 06:14 PM
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As you can see, i've been rather busy with the car today. I decided to take it all apart to see exactly how it worked.
Where the two arrows point both had short lengths of pipe attatched when I got the car, with stoppers over the ends. It looked as if a pipe used to join the two ports together, but now after taking it all apart, I can see how it works. If I connect both ports together, then air is allowed past the throttle plate and hence the higher idle.
Should there be a check valve inline with the pipe connecting the two ports? Otherwise, what gets connected to them?
Kind Regards
Ben
Where the two arrows point both had short lengths of pipe attatched when I got the car, with stoppers over the ends. It looked as if a pipe used to join the two ports together, but now after taking it all apart, I can see how it works. If I connect both ports together, then air is allowed past the throttle plate and hence the higher idle.
Should there be a check valve inline with the pipe connecting the two ports? Otherwise, what gets connected to them?
Kind Regards
Ben
07-28-2006, 06:48 PM
#7
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Ben,
You have been busy! You have discovered a throttle bypass route which if connected or left open to air would increase the idle just like opening the throttle. On my car there is an auxilliary air solenoid attached to the posistion on your 1st picture; it opens and sends air into a y-fitting which probably leads to the port in your second picture. It opens when the a/c goes on to blip the throttle to compensate for the compressor load. I'm not sure if you have an auxilliary valve to increase idle during warm up or if you have a idle stabilizer valve to control idle speed, but you have one or the other. These can be sources of vacuum loss if not connected or working properly. By the way if you really close off the ports completely in your pictures, it seems they should no longer contribute to high idle.
You have been busy! You have discovered a throttle bypass route which if connected or left open to air would increase the idle just like opening the throttle. On my car there is an auxilliary air solenoid attached to the posistion on your 1st picture; it opens and sends air into a y-fitting which probably leads to the port in your second picture. It opens when the a/c goes on to blip the throttle to compensate for the compressor load. I'm not sure if you have an auxilliary valve to increase idle during warm up or if you have a idle stabilizer valve to control idle speed, but you have one or the other. These can be sources of vacuum loss if not connected or working properly. By the way if you really close off the ports completely in your pictures, it seems they should no longer contribute to high idle.
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07-28-2006, 06:51 PM
#8
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Go to
https://techinfo.porsche.com/techinf...86_KATALOG.pdf
and page 50.
The hoses that are cut go to item 44, which is an air diverter valve only used if you have air conditioning, so if its plugged its not part of your problem.
I would for a start go through all the plastic Y anx X connectors in the vacuum lines and check, they crack and break very easily.
Also, as you are in UK, go by your local halfords or similar and get a vacuum/compression gauge kit. You can plug it into the vacuum lines with a Tee connector and learn a lot. I have one I bought back there almost 30 years ago, and it is still the easiest way in my opinion to spot a leaky vac line.
Good luck
Neil
https://techinfo.porsche.com/techinf...86_KATALOG.pdf
and page 50.
The hoses that are cut go to item 44, which is an air diverter valve only used if you have air conditioning, so if its plugged its not part of your problem.
I would for a start go through all the plastic Y anx X connectors in the vacuum lines and check, they crack and break very easily.
Also, as you are in UK, go by your local halfords or similar and get a vacuum/compression gauge kit. You can plug it into the vacuum lines with a Tee connector and learn a lot. I have one I bought back there almost 30 years ago, and it is still the easiest way in my opinion to spot a leaky vac line.
Good luck
Neil
Last edited by neilh; 07-28-2006 at 07:20 PM.
07-28-2006, 06:53 PM
#9
Drifting
Great progress! Now spray it all except the idle/wot switch thing(black plastic) with carburetor cleaner until there is no dirt at all including inside the throttle body, get the butterfly nice and clean and working right.
Top pic- arrow points to place where hose would connect to AC switch thing that hangs off engine brace bar. I ditched my AC and so I plugged that too. The top little hole is the steel auto tranny line connection.
bottom pic- the arrow points to the hose that connects to the oil pour in place. the connection on the right goes over to the y connector between the spider legs that would lead to the brake booster.
I would have to look at my engine for confirmation but its covered right now and raining. Time for the WSM on CD from morehouse for you.
While your at it clean the valley of all crud and anything else you can get at, O-rings on MAF, etc. Post pics of your progress and I will try to help.
Top pic- arrow points to place where hose would connect to AC switch thing that hangs off engine brace bar. I ditched my AC and so I plugged that too. The top little hole is the steel auto tranny line connection.
bottom pic- the arrow points to the hose that connects to the oil pour in place. the connection on the right goes over to the y connector between the spider legs that would lead to the brake booster.
I would have to look at my engine for confirmation but its covered right now and raining. Time for the WSM on CD from morehouse for you.
While your at it clean the valley of all crud and anything else you can get at, O-rings on MAF, etc. Post pics of your progress and I will try to help.
07-28-2006, 06:58 PM
#10
Drifting
Like Neil says replace most everything you can and then when you put it back together cap the maf with saran wrap and pressurize the system and listen for any air escaping. Did mine with an electric bike pump. Better if you use smoke.
07-28-2006, 07:10 PM
#11
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Ya sure you want to saddle yerself with that forum name forever, Ben? I'm bettin' you'll be around for awhile...
Definitely sounds like a vacuum leak problem.
Definitely sounds like a vacuum leak problem.
07-28-2006, 08:01 PM
#12
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Hi!
Well thanks for all the replies!
Its all starting to make sense now... one of the previous owners removed the AC, so that would explain the blanked off ports.
Great website with the diagrams on as well, thanks.
The pictures are of the throttle body after I cleaned it with carb cleaner! It was disgusting inside, caked on oil, and the butterfly felt as if it was grinding. I'm afraid to use any abrasives on it incase it doesn't seal afterwards. There are no solids/caked on stuff now, its just a bit of a stain.
I've cleaned the valley out too, looked like a mixture of sand and oil residue was in there. All three of the 'cotton reel' mounts broke, so I took a trip to my local Audi dealer, and got some that looked almost identical.
Re. the vacuum leaks... when I was taking it all apart, some of the hoses just pulled off without undoing the hose clip. I'm planning on using worm drive clips instead of the original type, so I can get a bit more pressure on them. Also the Y vacuum connector you can see in the second picture was barely on because the rubber had expanded over the port. I cut off the bad 1/2" or so and it now fits snug.
I connected a new piece of vacuum hose onto where all the X and Y connections originate from and gave it a good suck and put my tongue over the pipe. It held vacuum for as long as I could bare the petrol vapours in my mouth!
Looking at the diagram on page 50 as neil mentions, I dont recall seeing item 17 and its associated pipework.
And finally, going back to my original post, why would unplugging the throttle switch, idle air valve, and MAS not cause any difference in running? (maybe I should see if it runs good after I put it all back together tomorrow before I start worrying about this)
Kind Regards,
Ben
Well thanks for all the replies!
Its all starting to make sense now... one of the previous owners removed the AC, so that would explain the blanked off ports.
Great website with the diagrams on as well, thanks.
The pictures are of the throttle body after I cleaned it with carb cleaner! It was disgusting inside, caked on oil, and the butterfly felt as if it was grinding. I'm afraid to use any abrasives on it incase it doesn't seal afterwards. There are no solids/caked on stuff now, its just a bit of a stain.
I've cleaned the valley out too, looked like a mixture of sand and oil residue was in there. All three of the 'cotton reel' mounts broke, so I took a trip to my local Audi dealer, and got some that looked almost identical.
Re. the vacuum leaks... when I was taking it all apart, some of the hoses just pulled off without undoing the hose clip. I'm planning on using worm drive clips instead of the original type, so I can get a bit more pressure on them. Also the Y vacuum connector you can see in the second picture was barely on because the rubber had expanded over the port. I cut off the bad 1/2" or so and it now fits snug.
I connected a new piece of vacuum hose onto where all the X and Y connections originate from and gave it a good suck and put my tongue over the pipe. It held vacuum for as long as I could bare the petrol vapours in my mouth!
Looking at the diagram on page 50 as neil mentions, I dont recall seeing item 17 and its associated pipework.
And finally, going back to my original post, why would unplugging the throttle switch, idle air valve, and MAS not cause any difference in running? (maybe I should see if it runs good after I put it all back together tomorrow before I start worrying about this)
Kind Regards,
Ben
07-28-2006, 08:27 PM
#13
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Interesting, the page i linked you to was US spec, not rest of world. Maybe someone in UK has the correct link for the ROW ( euro's).
17 here is the thermo valve controlled by the block temp at the rear of bank 1-4. If it is missing, do you have 26 - 30, and how are they connected? This could explain your problem.
BTW: Brett Ainley runs the UK 928 board, do a search for him on here and you will get his email, he's a good resource over there.
Cheers
Neil
17 here is the thermo valve controlled by the block temp at the rear of bank 1-4. If it is missing, do you have 26 - 30, and how are they connected? This could explain your problem.
BTW: Brett Ainley runs the UK 928 board, do a search for him on here and you will get his email, he's a good resource over there.
Cheers
Neil
07-28-2006, 08:40 PM
#14
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With new hoses you shouldn't need big time clamping forces. The old hoses get mushy from oil exposure. With new hoses just use the standard clamps.
07-29-2006, 06:11 PM
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Thanks to all who've helped.... Its working. Nice smooth idle @ around 800 rpm. I can now shift into drive without it stalling too!
I resealed basically everything on the intake, all gaskets, new pipes where needed, new hose clamps. I changed the fuel return pipes too, along with the coolant pipe that runs under the plenum while I could get to it easy.
Is the screw on the MAS to adjust the CO level? It looks messed about with, so i'll hook it up to my CO analyser and see if its out.
Can you tell i'm happy now that it works?
I resealed basically everything on the intake, all gaskets, new pipes where needed, new hose clamps. I changed the fuel return pipes too, along with the coolant pipe that runs under the plenum while I could get to it easy.
Is the screw on the MAS to adjust the CO level? It looks messed about with, so i'll hook it up to my CO analyser and see if its out.
Can you tell i'm happy now that it works?