Problem with Engine Idle RPM

I just had my car in the shop to repair a broken clutch engagement arm fork (sorry, I don't know the part's technical term). As with all clutch work, the engine had to be lowered. In putting back the engine connections, the mechanic told me that he had to adjust the throttle linkage because I was not getting full throttle. After driving the car for the last few days, I noticed that the car was idling at about 1200 rpm for a while after starting, but after extended use at high speeds, the idling rpm was up to about 1600 to 1700 rpm. When I turn the air conditioner on, the idle speed comes back to about 1200 rpm and when I turn it back off the idle speed goes back up to 1600 to 1700 rpm. I called my mechanic today and he told me that the idle adjusting screw was already in as far as it can go and that I have to drop the car off for a couple of days for him to find out what the problem may be. When I picked up the car last week, after he had fixed the clutch problem, he told me that the vacumm advance was not working properly and that some of the hoses and connections did not appear to be in their right configuration. Before being converted to CIS by its previous owner, my car had Webbers and hot cams. Can someone tell me what the problem I'm experiencing might be and how I can adjust the idle to operate at about 1000 rpm. I would appreciate any feedback. Thanks

I have some doubts about your mechanic. Perhaps he (or she) is very experienced and honest...I'm not calling anyone names here but I have doubts.

First, my CIS throttle linkage is composed of rods, not cables. I'm not sure what your level of comfort under the hood is, but I'd first check that throttle linkage. It should have some looseness to it. the throtle linkage should should not be pulling on the throttle at idle. The looseness should verify that the throttle body returns all the way to the 'rest' position. If the linkage was shortened, then it might still be holding the throttle plate open at rest.

While I am making no accusations here, i'll point out that throttle linkage rods are easy to bend, particularly when removing an engine. Once bent, they are difficult to adjust and make them work right again. If bent, they might very well be holding the throttle open at idle, causing high idle.

If the idle adjusting screw is turned all the way in, then my suspicions (above) about bent rods and/or linkage that is holding the throttle open at idle becomes more than a suspicion. At idle, the throttle plate should be (I believe) completely closed, and the idle adjusting screw should be backed out far enough to allow air to bypass the (closed) throttle plate.

Ask questions if you don't understand this, Ruben. For all you others, if I'm missing something here please enlighten us. My knowledge is more 'general' than "Porsche," as many of you know.

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'83 SC

Superman, thanks for the reply. One more piece of information and a couple of more questions. First, my mechanic told me that he had to reposition and weld one of the throttle linkage rods because the car was not achieving full throttle. What I can't explain is why when I start the car and during the first several miles, the rpm is at about 1200 rpm and is not until I have driven the car at higher rpms for a while that the throttle settles at the 1600 to 1700 rpm mark. Also, can I get to the throttle linkage easily to see if the throttle is not coming back to idle position fully. In which part of the engine compartment do I look to find the linkage and the idle addjustor screw. I apologize for my ignorance, but this is my first 911 and I have only had it for several months. I intend to become more knowledgeable with the mechanicals soon. Thanks

Ruben
1976 911 2.7L

Ruben,

Let me get this straight, OK? Were you having any idle or sticking throttle/response problems before you took the car in for clutch work?

If not, then to use a medical analogy, I think you have a case of malpractice in the care of a sick Porsche on the part of a so-called mechanic!!! The only logical reason I can think of for your mechanic to have to weld up a throttle linkage rod would be if he broke it in the process of removing the transaxle and engine! It could conceivably happen if he lowered the engine and transaxle while the throttle cable was still attached to the throttle rod and bellcrank on the side of the transaxle!!! If the bellcrank is bent out of shape, and welded upon, or likewise for the bellcrank end of the throttle cable, then it seems likely and probable that the belcrank was torn from its' pivot pin on the left side of the 915 transaxle ... a digital pic of that area would tell volumes about the malpractice heaped upon your poor Porsche! I hope your situation is not quite as bad as my imagination is ... but I have this 'FUNNY FEELING' I'm not wrong about what happened!!!

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Warren Hall
1973 911S Targa


[This message has been edited by Early_S_Man (edited 08-31-2000).]

Ruben, my car is an '83 SC with CIS. I'm going to assume yours is similar. The part of your car that Warren is talking about is underneath. The transmission is cone shaped pointing forward, and a rod runs along site it on the driver's side. The rod goes up toward the top of the engine. The other end is (I believe) joined to another rod sticking out of the car chassis. I'm not exactly sure what holds the rod steady but it would be a part attached to the tranny. Warren's concern (I think) is that even the transmission case may have been broken! Look to see if the welding was done in that area, and whether it was done to the brass colored rod, or to the silvery cast metal transmission casting.

In the engine compartment there is a small-breadbasket sized plastic box atop the engine, busy with hoses and such. The throttle body is attached to that air box on the driver's side, near the centerline of the motor. The brass rod is coming up from the tranny to a bracket. The rod pulls on the bracket and is attached to it. But it should be attached loosely. It should rattle at the joint. The bracket should come to it's own 'stop,' rather than being held in position by the rod.

If you reach your had a little further in, and above, there is a big, nickel sized screw head, also on the throttle body. This is the idle adjust screw. My advice is to not fiddle with this too much yet, but I'd be interested to know if it's all the way turned in one direction or the other. I'd also be interested in whether the engine speed changes when you turn it.

You need to find those welds, though. You're scaring Warren.


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'83 SC

Ruben,

Find another mech. Do not go back to this Liar. Think about this... Porsche does not build cars that can not be opened up to full throttle.

He broke it just as Warren said (I also can picture this Goober in action in my head) then he lied to you about it.

s

Well, look under there and see what was welded. I looked and there is a rod 1/2 to 3/4 inches in diameter and maybe five inches long stocking out of the tranny. It's clearly visible when you take the rear tire off, for example. The rods (from the chassis and from the engine) are attached to it. If this thing broke....well, let's just find out what happened. Find the welds. Find 'em now while they're bright, shiny welds.

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'83 SC

I would urge you to determine what the mechanic did to your linkage as soon as possible, before your throttle sticks wide open! There are many places to adjust the linkage on my 911 - it is hard to imagine why someone would need to cut it apart and weld it together again.

If you are unfamiliar with your 911, have a more experienced owner or a different shop look at it - soon.

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Bob Thayer
1971 911T
rthayer@hdrinc.com

Having had a lot of trouble in this area myself, I'll try and describe the throttle linkage on my '73 CIS 911.

Starting at the gas pedal, a short rod on the underside of the pedal connects to a crank on the pedal cluster.

NOTE: The gas pedal itself is actually separate from the brake and clutch pedal assembly - it screws directly into the floor.

A ball type joint connects this pedal crank to a long metal rod, about twice the thickness of a wire coat-hanger. This long rod goes all the way down the central tunnel of the car. It is supported by 3 nylon or plastic bushings:

1) at the pedal end of the tunnel.
2) One near the emergency brake (central area) of the tunnel.
3) One at the engine end of the tunnel.

All the above bushings fit into sheet metal collars, and all (on my car) were broken and/or missing.

The metal rod also has an adjustable metal collar for thge handle throttle to push against.

The end of the long metal rod connects to a crank on the transmission case. At first glance its easy to mistake it as part of the clutch mechanism!

From this crank another metal rod passes up into the engine bay at an angle of about 50-60 degrees.

This angled rod connects to yet another crank, at the forward end of the engine case. This crank has nylon/plastic bushings which (guess what) were broken and/or missing on my car.

From this crank a short metal rod connects to the CIS unit itself. A spring is attached here that closes the throttle when no pressure is applied to the pedal.

Quick and "dirty" fix:

Try replacing the throttle return spring with a stonger spring. Take the existing spring out and compare to a series of throttle springs - you are basically looking for slightly shorter length and/or slightly wider coils.

TIP: I used a spring for a Jaguar 3.8 straight six engine with carbs - worked a treat!


Correct fix:

Obtain three center tunnel bushings and two throttle crank bushings.

Jack up the rear of the car (or drive it up ramps).

Remove front carpeting/mats/pedal board.

Remove rods from centre tunnel and engine bay - check for straightness, relubricate, and renew bushings (if necessary).

Grease all linkages and cranks and what-have-you.

Your throttle pedal will now have mm precise accuracy and driving your 911 wil;l be a lot more enjoyable.

Time for the above: 3-5 hours.

Good luck!

- roGER

Ruben, you're not responding and, maybe I'm just anxious to hear what the problem is, but I'm also concerned about you. Don't freak out, and also don't take the car back and argue with the mechanic, at least not yet. If anything, take it to another mechanic who has no interest in the matter or familiarity with the mechanic that did the job.

You MUST find the welds. Find 'em while they're fresh and will photograph well. Again, don't freak out. This may have a simple fix. But if Warren's fear turns out to be the problem - if the thock rod sticking out of the transmission case has been broken off and welded, then repairing the damage will cost the mechanic (or you) lots of money.

You don't even have to take our advice, but we're trying to help.

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'83 SC

Hey guys thanks a lot for the feedback. I just got back from work in the 911 (my daily driver) and the problem is still there. As long as I keep the air conditioner on, the idle stays at about 1200 to 1300 rpm. When I turn the air off and keep the engine running, the idle speed goes up to 1700 rpm. I will not be able to look into it tonight but will defenitively get to it tomorrow. I appreciate all the suggestions and will get back to you guys with what I find out.

Thanks
Ruben 1976 911

I understand being 'busy.' I'm anxious to hear what's happening though.

You reminded me of something I forgot to say earlier. The A/C will drain horsepower, and rpms, from your engine, so the lowered idle with the A/C on is not surprizing. Further, warm engines idle better (higher) than cold engines. This is all consistent with my reading of your symptoms. there are fast-idle mechanisms designed to keep the idle up during cold starts and during A/C operation, but they are not having an effect on your idle, it seems.

I still suspect that this is because your idle is being set by the tension put on your throttle plate by the rod coming up from the tranny. While I am most interested in learning what was welded, I am also curious as to whether the throttle body mechanism is really coming to rest or not. I looked at my throttle body and it is not hard to see and access. The rod coming up from tranny, and connecting to the throttle body linkage wiht a ball-and-socket joint, shoud rattle at that joint a little. It should not be pulling on the throttle body part, holding the throttle plate (not visible) open.

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'83 SC

Hi Guys, just got back from inspecting the throttle linkage. I got under the car and did not see any indication of anything that was broken and welded back together again. The rotating throttle section which is attached to the tranny appears to be intact. The linking rod which pulls on the throttle was not tight as I was able to rotate it back and forth a bit. This was true even after I had run the car for a while and the idle rpm was at about 1700 rpm. The throttle lever also appears to go back to the idle position when the car is rest and the accelerator is not depressed. I also confirmed that the idle adjusting bolt was in as far as it can go. When I unscrewed it the idle rpm went up. At this point I don't think the problem has anything to do with the throttle control linkage. My next theory is that maybe something was not connected properly. I looked at a fuel injection diagram Porche Parts and Technical Catalog which shows the connections, but I got lost when some of the equipment shown on the diagram did not match exactly the setup in my car. Any other suggestions which I can look into which might explain my problem. Thanks for all the feedback.

Ruben
1976 911 2.7L

Whew!

Okay, I think we still need to hear from the mechanic or by some other means determine what welding was done.

Good investigating. The throttle body has a big round "butterfly" valve that IS the throttle. At idle I believe it should be closed. Of course, as the gas pedal is pushed, the rod pulls and the butterfly valve opens, and this makes your car go.

The air allowed in during idle actually goes through a bypass tube that is partially blocked by the idle adjust screw. Bypasses the (closed) butterfly throttle. With the screw all the way in, I don't know how it's getting air.

Here's another factoid. The air comes in the big black thing with the horn and then it travels across the smaller but still big rubber thing at the VERY TOP of the air box. Just to the right of this rubber thing is a black cast iron octopus (or spider if you like) with fuel lines sticking out the top. This is the Fuel Distributor. Mine are metal and yours may be flexible hose. the reason I tell you about this is that air MUST be going past the plate that is under the black rubber thing and just next to the octopus. Air must be moving past that plate, displacing it, because that is what tells the fuel system to work.

So, idle air is getting past the sensor plate. I wonder.

I'm glad yoru tranny case is fine. I gotta go, but I'll consider your symptoms. Someone else will probably figure it out before I return, but I sure wonder where those welds are, if he said he welded something.

Did you pull back on the linkage mechanism at the throttle body with the engine idling. That is, did you try to force the idle to slow down by moving the thing on the throttle body that the rod hooks to? If you gently push it that way, it may change the idle.

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'83 SC

Ruben,
There are a few other devices which control
idle RPM directly:

1) "auxilliary air device" located just forward of the idle adjust screw and where the throttle linkage connects to the throttle body between the engine and firewall. It has a vacuum hose going into the top of it. It's purpose is to control backfiring on decelleration, emissions on decelleration, and keep the idle high for a second upon deceleration. If it's stuck open it'll cause your idle to be way too high.

2) "Auxilliary air valve" located on the passenger side of the engine forward and below the fuel distributor between the engine and firewall. It's purpose is to control fast idle speed when the engine is cold in conjunction with the "auxilliary air regulator".

3) "Auxilliary air regulator" located on the passenger side of the engine in between two of the intake tubes. it works with the valve above (they're connected) to control fast idle when cold. If stuck open will cause idle to stay too high.

All of these componemts have vacuum lines going to them, some big, and some small which could have been connected improperly, and in the case of the regulator it also has an electrical connection for an internal heating element.

I emailed you a picture showing where these parts are located on your engine in case you didn't know.

Leon
78SC

[This message has been edited by leon (edited 09-02-2000).]

Well, I re-read your first post, Ruben, and I see no mention of the mechanic welding stuff. I must be going crazy. I did see that there is already a suspicion of hoses routed improperly.

Leon's right, there are several air handling devices that bypasss the throttle. Any air getting into the larger hoses can increase idle. These devices will be basically off after the car is warmed up. With a rag and a pair of pliers you could squeeze them. They should have no effect in theory. Do you agree with this, Leon?

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'83 SC

Superman, you are not going crazy. In an earlier post I did mention about the mechanic telling me he welded something on the throttle linkage mechanism. However, I could not see any new weld on the mechanism when I checked it this morning. I intend to call him on Tuesday and find out exactly what he welded. I think the idea of testing with the pliers to seal off any air that is still present sounds good. I have a picture that Leon sent (Thanks Leon) that shows the location of the auxiliary air valve and device, ant the auxiliary air regulator. I will try to do that tomorrow, unless someone thinks this may damage something. I'll check the board tomorrow before I do anything just in case. I appreciate all the feedback. Thanks

Ruben
1976 911 2.7L

Superman,
I agree with you. I guess Under normal circumstances you could do a quick check of the Auxiliary air device, and the Auxiliary air regulator that way. However, in Ruben's case because the problem is probably not with the devices themselves but with the plumbing connected to them (they worked before), I think he should verify correct plumbing to them in addition to testing them. For instance, if one of the bypass hoses going to the Auxilliary air device is cracked, broken, or improperly connected. Pinching the hose would also have no effect.

Ruben,
If you suspect the throttle linkage you could pop it off at the swivel joint where it connects to the actuating lever at the throttle body. Just give it a good pull and it should pop off. This will effectively take the linkage out of the picture. Then start the car. If the idle remains high it's not the linkage, if the idle goes back to normal you know it's somewhere in the linkage.
I emailed you the procedures out of the shop manuals for testing the various air controlling devices, along with my own notes. Also, there's a plumbing diagram showing how everything should be connected. Let us know what you eventually find.

As they say in Japan,
Rotts a Ruck,
Leon 78SC

[This message has been edited by leon (edited 09-03-2000).]

Well, Leon apparently has a good grip on this system. I also am concerned that a vacuum hose, which operates one of these devices, may not be routed properly. And a cracked big hose may still leak air after pinching, depending upon where the crack is. Still, if pinching a hose while the engine is up to temperature has an effect, this would be a clue. chances are, with the idle screw all the way in pinching one of those hoses is likely to make the car die.

Ruben, I'm glad to hear taht the welding comment I remembered does in fact exist, but I'm probably still going crazy. At least that's what I suspect Warren may be thinking. A smilie would go here if i knew how.

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'83 SC

Sorry for not posting something sooner but my home PC took a dive this weekend and is out of service at the moment. Between the PC and weekend activities, I did not get to do much on the car. I did try blocking the flow from the auxiliary air valve and this did not make any difference (there was suction in the line). I also tightened one of the hoses on the air regulator valve which was a little loose and this also did not make any difference. If I get a chance, I intend to do some more checking tonight when I get home. I'll keep you posted of what I find out. Thanks

Ruben
1976 911 2.7L