Bosch MFI excessively high idle

[BURN-BROS]

Originally Posted by McClellr

Today I double checked the heater hose and found a small leak. Fixed that, but no change.
I tried pushing the rack through the front access (again) to see if that was sticking. It feels free and while it decreases speed as I push it in, it snaps back to the same position and speed.
I noticed one thing interesting. My 114mm shaft to the fuel pump is long enough to slightly open the rack when attached. When I pop it off, effectively closing the rack, the speed increases. This sounds like Im running rich at my 2000 rpm idle. I can also increase speed by opening the throttles with the pump disconnected. I might be able to force it down with more fuel but, goodness, what a way to tune an engine. Glad we killed emissions checks in Michigan.
It seems like Im just getting too much fuel. I dont have an A/F meter. I tried leaning the main fuel screw two clicks, with no change, but not richer. I can try that and I can get a meter, but I think I know I am way rich.

None of the things you mention above help to diagnose of remedy your issue. The idle will DROP when you enrichen the mixture. Vac leaks lean the engine out, raising the idle.

Originally Posted by McClellr

The throttle bodies all rest on their stops. The shaft to the pump (114mm) opens the rack slightly. But that isnt my problem. Ive run it with that disconnected.

so adjust the length of the 2 crossbar drop links that attach to the t-bodies untill your pump is at it's stop.....it will not solve the problem but why not make it right?



You need a carburetor sync, a way to measure mixture, and you need to map your distributor curve.......If you do not do/get these things you will most likely never fix your problem unless you get professional help.

[McClellr]

I have the carb sync and have used it. Out of frustration, I set the air correction screws all at a medium balanced setting and the throttle stops at minimum. I will get back to that if I ever get control of idle. I have effectively mapped the distributor curve with strobe and fuel shut-off. I could draw a graph, but dont know what I would do with it. I know timing is correct at no advance and max advance and doesnt seem to hang up. Thanks for the suggestion to adjust the drop links. I will do that tomorrow.
Im curious why you suggest that increasing fuel flow is the way to decrease idle. While it may work in an already rich condition, in a perfect world, the mixture would stay stochiometric or close to it and fuel control would be speed control. Does the MFI always run super rich at idle? What would the AFR read if things were as designed?

[McClellr]

I called Otto's in Venice CA about the RPM transducer. He said send the sensor and $175.00 and he would send me a good used one. Is this the best price? I would prefer someone who can test it and tell me that it is in fact defective or not. What if the failure is elsewhere?

[Flieger]

Shake the box. Does it rattle? If so, you probably have a transistor with a broken lead. Open the box and look at the board.

If you have access to some electronic equipment like a bench power supply, and have or can make a suitable distributor signal simulator, you can fix it yourself.

MFI runs rich at idle. Usually moderately rich. That is what gives the initial throttle response to raise the revs when the throttles open and allow more air in.

[tobluforu]

Originally Posted by McClellr

I called Otto's in Venice CA about the RPM transducer. He said send the sensor and $175.00 and he would send me a good used one. Is this the best price? I would prefer someone who can test it and tell me that it is in fact defective or not. What if the failure is elsewhere?

Well I guess things have changed as it cost me 95 bucks for one that he rebuilt. But before you do, bend the tabs back and pull it apart to check.

[BURN-BROS]

Originally Posted by McClellr

I have the carb sync and have used it. Out of frustration, I set the air correction screws all at a medium balanced setting and the throttle stops at minimum. I will get back to that if I ever get control of idle.

This is 50% of idle quality. The flow quantity is paramount to idle control. Worn shafts and damaged bores pull too much air around the plates. What is a "medium" setting? What # did you set it to?

Originally Posted by McClellr

I have effectively mapped the distributor curve with strobe and fuel shut-off. I could draw a graph, but dont know what I would do with it. I know timing is correct at no advance and max advance and doesnt seem to hang up..

post your findings....what rpm does advance start?

Originally Posted by McClellr

Im curious why you suggest that increasing fuel flow is the way to decrease idle.

I misread your post.

[jpnovak]

I would start looking for air leaks. This could be under the throttle body, the injector mounts, air bleed screws or excessively worn throttle plates/bores.

Have you verified that all throttle plates are closing down fully?

I would take off the throttle bodies and look at them behind a light. Does there appear to be much wear? Open the butterflies and feel for ridges or open spots. Do the throttle shafts have play?

If you know your timing is correct and you know that you are not running really lean you need to look further.

[McClellr]

I opened the RPM transducer. There are six transistors. All are intact. In fact the unit looks brand new. Not a sign of corrosion, dirt, anything. I put the ohm meter on the transistors. Not my strength, but if one were blown, I think I would have found it. I only get a momentary signal.
Im going to double check with a transistor attachment as soon as I remember where I put it.

My air correction screws are all set at two turns CCW off bottom. Throttle stop screws are at minimum that allow butterflies to rest on them.

Ill rerun my ignition advance test. Need another set of eyes.

[Flieger]

If the RPM transducer worked before and there are no broken transistors, your box is probably fine. You said you blew one of the engine bay fuses. Did you replace it? I would think it would work again.

You know, I like my idle to be up around 1500 or so. It is smoother, probably compensating for reversion and worn throttles. How much do you idle, anyway? 2000 rpm is not so bad. If you run into speed switch occilation with that, you can tweak the limits for the fuel shutoff in the RPM transducer. This takes a potentiometer, a singal generator (distributor simulator), and a scope. I had a bunch of way old, out of calibration stuff I got from some elctronics warehouse. Put a 12V offset DC square-wave to replicate the points signal. Then vary the Hz to change the RPM. You can see when the relay is open/closed on the scope. If you vary the resistance in the potentiometer, you can change the lower limit.

This is all detailed in the thread I linked.

I would rather have a high idle than go through the trouble and expense of removing and rebuilding the throttles at this time.

[jstobo]

i just saw this thread and haven't read all of it but here are some ideas. If you have already tried them then I apologize for being lazy and not reading the whole thread.

Your RPM transducer will not effect the normal idle. It may be bad but it will not change the idle adjustment. It just shuts off fuel when the throttle is closed and RPM's are over 1500 RPM. And restarts fuel when Rpm drops below about 1000.

The throttle bodies air screws set the idle on MFI. You must match flow of each cylinder. The idle is only set by these screws. If they carbon up spray Chemtool B-12 (per CMA) to clean. If you cannot set the idle with these screws you are Screwed(he he). If clean then you probably have air leaks and may need some throttle body repair.

BE SURE that the idle is not set by the screw that depresses the micro switch. Unscrew the micro switch screw completely when setting idle. After adjusting idle, screw in just enough to hear the click with the throttle closed.

Contrary to popular opinion the MFI ran pretty lean at idle when new. Just check the CO amounts in the manuals. But my guess is that through wear, adjustments have to be made to get them to run through all RPM ranges. Also modifications of the engine will ruin the MFI exactness as each pump is tuned to the volumetric efficiency of the engine it was designed for. So engine modifications require more compromises. Gus at Pacific told me if you modify your engine to a Porsche spec engine he could set the pump for that engine. But otherwise it would just be a compromise.

Good Luck.

[McClellr]

Yes, I replaced the fuse and tested it with a an ohmmeter. I know it was working before the fuse blew because it was having a gay time with the high idle. I had to disconnect it.

Definately got the microswitch out of the way.

One thing that leads me to look for something out of the ordinary is that when I go out tomorrow, the engine will start quickly and idle at a low idle for five minutes or even longer. I can run it up and it will fall back down to low idle, no problem. But after several minutes and it warms up, it will run up to 2000 and that's it. When it decides to go, it is like something changed. It will be at a few hundred rpm and then just climb right up. I dont have to touch it. I can precipitate it ,however, by running the speed up.

I dont believe the pump is original to the engine because it doesnt have the fuel enrichment solenoid. But it runs fine at speed, at least to my satisfaction, even if the urve is a bit off. Seems like idle shouldnt be much affected and it did work when I got it.

[Flieger]

I saw John Dilger's 1971 911S racecar do that very thing as he was warming up the car in the paddock to go out on track. He was not even in the car- getting his race gear on. My Dad and I were worried it would run away to redline.

Perhaps there will be some more ideas on the Early 911S Registry? Mark Jung has already replied here, though.

Early 911S Registry Bulletin Board - Powered by vBulletin

[Jeff Higgins]

Maybe something in the throttle linkage? What happens if you disconnect the throttle rod from the cross bar? I broke a transmission mount once, and the tranny squirming around on top of would pull the throttle open a bit.

Or, even weirder, how about the thermal time switch and the bypass solenoid? Maybe the thermal time switch is shorted internally and it sends a signal to the bypass solenoid on top of the filter console to shoot (or maybe just dribble) a priming charge. Pull the lead off the bypass solenoid and see what happens.

[boxster03]

Grady he needs you

[McClellr]

I disconnected the cold start that goes to the air filter housing just on principle. What an idea on top of such a sophisticated system. Plus Im doing much of the work with the housing off. The engine starts readily.

I have run with the linkages disconnected multiple times.

[356RS]

I would really like to figure this problem out. I lot of help here and you have checked almost everything you can with the tools you have. I have read all the posts and keep going back to the fact that your car runs fine, starts cold, runs great warm but ends up with idle at 2000. A lot of posts suggest TB's, linkage and or air/vacuum leaks. This would diffidently cause the idle to go up after warm up. The odd thing is that it seems to happen very fast at a point during warm up if I'm understanding your posts correctly. Is there something happening in the thermostat during warm up? I don't think so. Any problems inside the thermostat will either keep it from extending and run very rich, or after warm up get stuck extended and never return for cold start. Or is the thermostat lever to balance bar inside the pump causing some problem? Is there something inside the MFI pump that can all of a sudden during warm up take things up to 2000 rpm at idle? Lots of possibilities, no answer. There is one thing that you did that brought the engine back to idle. You pushed on the main rack, giving the system more fuel and it seemed to like it. Was this added fuel masking some air/vacuum leaks? I made a suggestion toward the beginning of this to take a few turns CCW on the main rack. What I wanted to see was #1, will it now keep a low idle? #2, same thing happens again but with a little lower rpm instead of 2000. Doing this will help on figuring the problem out. It might help to eliminate the pump. Give it a try, what can you loose.

p.s. If you do this adjustment on the main rack, do it after warm up and try 6 clicks CCW.

[al lkosmal]

Do you have the hose to the thermostat connected? I defer to the MFI experts that have already checked in, but this week, after many drives with perfect idle, I had a high idle issue. After warming up, my idle was rising to 2500 RPM or so. I found two things amiss. My 1,2,3 linkage rod was slightly too short, possibly influencing the idle speed. After fixing that the idle dropped from 2500 to 2000. As I was fixing/checking the linkage, I noticed that the hose to the solenoid had slipped off. I re-clamped it to the solenoid and the problem seems to have gone away.

Regards,

Al

PS: If I'm thinking of this correctly, the thermostat acts as part of the cold start mechanism, providing an enriched mix. If the hose is on and functioning the warm air from the exchanger heats the thermodiscs, causing them to expand and change the fuel accordingly. Will the hose coming off, cause the thermodiscs to cool and retract and assume the "starting" position again and will this cause a high idle?

[McClellr]

The hose to the thermostat is fine. I took the top cover off the thermostat and watched it work with a hot air source (not the engine) It pushes right forward and looks like it does its job. I didnt actually do it with engine heat. Maybe I will tomorrow.

I have been through the linkages and adjusted them. I also have run with them all disconnected.

Today I took the sync and identified the throttle bodies that were moving the most air. I adjusted them down more carefully, both stops and air screws until they were at the least I can get without resting the butterflies on the bores. I succeeded in getting the idle down. But it was running horribly rich. I could see it in the exhaust and when I disconnected the fuel pump linkage and just opened the throttles, it took off like a cat out of a cage. Honestly, I think I could drive the car on throttles alone. I am getting way to much fuel at idle.
I will try turning the main screw richer as advised, but I think that will make it worse. I havent tried turning the main screw more than a couple of clicks. I was a bit afraid of messing with main mixture, but I suppose it cant hurt the engine at idle. I think tomorrow, I will turn the main mixture screw all the way lean and see what happons. What do you think?
It acts like something causes it to advance the rack and turn the helix out of the idle range and on to the part load curve. When I give that fuel flow the air it wants, Im over 3000 rpm.
I have adjusted the idle mixture screw dramatically with no affect. I have the idle settings so low now that for the first about five minutes it idles like an old John Deere two cylinder. I give it some more fuel and air to keep the oil pump moving.

[Flieger]

Do you have any modifications to the system like a performace exhaust or a different air filter/plenum?

[tobluforu]

How are your fuel lines? Maybe after the car warms up one is constricting which is causing a lean condition????