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I need help getting the idle to settle down to a normal 800 RPM. It idles at 800 RPM when first started and slowly increases to 1500 RPM after a few minutes. The engine is a NA Bosch injection B230.
I have checked the ECT resistance at the ECU plug pins 5 and 13 and get 2500 ohms at 40-45 degrees F and 200 ohms at 195 degrees F (running temp.). That is about right according to the FAQs. The problem began shortly after I cleaned the IAC valve and the throttle body. The throttle body microswitch makes at idle and opens above idle like it should. I'm not sure what the 3rd wire on the microswitch plug does. I have double star washered the two brown wires on the intake manifold in case of bad grounds. I have wiggled the wiring around the microswitch and IAC valve and got no change in the idle.
Any ideas would be appreciated.
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Mine:3-940s running, 1-740, 2-940 parts, dtrs:3-940s running
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Dear jd620,
Hope you're well. I gather you've cleaned the idle air control valve (IACV), without effect. For idle problems, that a first good step: it costs only some time, energy, and a little throttle body cleaner.
As the Engine Coolant Temperature (ECT) sensor "checks out" - resistance is within specification - I'd guess the problem is elsewhere.
Perhaps the root of the problem is inside the IACV, or between the wiring harness connector and the IACV's electrical connector. A faulty or corroded connection to, or inside of, the IACV creates resistance to current flow. Resistance creates heat. Thus, at start-up, the IACV works normally, and then - as the faulty connection heats-up - slowly goes "off spec".
I'd begin by cleaning the IACV's electrical contacts and those on its wiring harness connector. A spray corrosion remover, e.g., Deoxit, will dissolve any corrosion. This takes little time and costs little money.
If cleaning electrical connections does not yield a stable idle at 700-800 RPM, it's time to take a used, "known good" IACV, and replace the in-service unit. This swap takes only a few minutes. If the "used, known-good" IACV, keeps the idle at 700-800 RPM, problem solved. If not, then the IACV is "exonerated".
Before spending on replacement parts, I'm partial to cleaning electrical connections and using "used, known-good" parts to verify component status. The Bosch IACV is not cheap.
Hope this helps.
Yours faithfully,
Spook
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I have cleaned the IACV twice and swapped it with a known good unit with no help.
This morning I received a new Bosch ECT and installed it. Same problem remains. The contacts on the IACV and ECT have been cleaned.
I am thinking the problem may be the ECU and will swap that out with a known good unit. Fortunately I have another 94 with Bosch injection.
The throttle body microswitch has continuity between two of the three wires at idle and drops continuity above idle.
The problem started after cleaning the throttle body and IACV. The car was running after the cleaning for maybe 10 minutes and all of a sudden the idle RPMs increased.
The car runs great other than the 1500 RPM idle.
Please, all suggestions are welcome!
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Mine:3-940s running, 1-740, 2-940 parts, dtrs:4-940s running
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The fix was common sense. The IACV can only do so much and will not be able to control the idle if the throttle body (TB) plate is open too much, which was my problem. Evidently, some previous owner had, instead of cleaning the dirty TB, cranked up the throttle plate stop screw to increase the idle RPM. So, when I cleaned the TB, the RPM jumped to about 1500 when the engine was warm since there was not a bunch of crud to block the air flow.
I adjusted the plate stop so the brass plate was closed but not touching the casting sides and re-adjusted the microswitch per the FAQS.
Idle is now a steady 750 RPM.
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Mine:3-940s running, 1-740, 2-940 parts, dtrs:4-940s running
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Dear jd620,
Hope you're well. Please apologize - on my behalf - to the idle air control valve (IACV), for my efforts to blame it for the high idle.
In your first post, you reported you'd cleaned the throttle body, checked the micro-switch, etc. So, I assumed that all was well with the throttle mechanism. If so, then the IACV was to blame.
In any event, glad the idle is where it should be.
Yours faithfully,
Spook
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Hi Spook, yes I did clean the throttle body and check the microswitch first but, alas, I did not check whether the throttle plate was closing all the way.
In the numerous throttle bodies I have cleaned and checked the microswitches, I have never adjusted the stop but that will be a check in the future.
The IACV can only do so much to smooth the idle. It must be able to override, allow more air to pass as needed, than can pass through the throttle plate. At least that is the way I view the IACV operation.
Thanks for your many good posts.
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Mine:3-940s running, 1-740, 2-940 parts, dtrs:4-940s running
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