My '88 535is idles fine when it's 'cold' outside: <80F.
From 80 to 100F when I depress the clutch pedal approaching a stoplight, the rpm dip to 400-500 for a couple seconds then go back up to the normal 700.
Yesterday, when my OBC showed 121F ambient, the rpm dips but does not recover and the engine stalls. She starts right back up.
WTF??? Any suggestions?
(an aside: How do male Tucsonans tell temp without a thermometer?
Length of the ol' ballsack. When its over 110, we need to wear knee-length shorts! )
Where was the temperature gauge when that happened? You might want to check the resistance (per Bentley) on the engine temp sensor. I would also check the AFM for worn traces and make sure that the voltage tracks smooth when you move the AFM flap. I know "Rich Euro M5" posted the AFM voltage check procedure recently.
I don't think I've ever seen 121, I think 116 or 117 is the most I've seen displayed on the OBC without any heat soak from being stopped.
Regardless, a couple of suggestions. Clean the idle control valve with carb cleaner, at those temps there is less air by weight, and it has to open wide quickly. So if it is dirty or gummy, it could be a problem. AFM is a possibility, I oval the holes to shift it slightly away from a worn track, others like bending the wiper so it tracks on a new spot. Make sure your throttle position switch is adjusted correctly. And lastly, is a real long shot. On my old E12, I found replacing the fuel pressure regulator cured the problem. I never tested it, but I did get stalling coming up to a stop at high temps. E12's do use a different FPR, but similar design, a diaphram with a spring and a vacuum connection.
Forgot to ask: why 9 volts instead of 12 to test AFM?
I would suspect it is because the 9 volt batteries are small and portable and you can get the clip on leads at any Radio Shack. The absolute voltage is not important, a smooth increase or decrease of the voltage related to flap position is. If I remember correctly on a 533 that citcuit is 0 to 12 volts, the later cars use a 0 to 5 volt circuit for the same input to the ECU.
Forgot to ask: why 9 volts instead of 12 to test AFM?
I would suspect it is because the 9 volt batteries are small and portable and you can get the clip on leads at any Radio Shack. The absolute voltage is not important, a smooth increase or decrease of the voltage related to flap position is. If I remember correctly on a 533 that citcuit is 0 to 12 volts, the later cars use a 0 to 5 volt circuit for the same input to the ECU.
Mark is correct. I'd rather do the test on a work bench vs in the car. Since most people don't have a lab type power supply, the good old 9 vdc transistor radio battery is a good alternative. In addition, it's current output is limited and won't burn anything up if you connect it incorrectly to the AFM. Hook a 900 cranking amp 12volt car battery incorrectly and you could easily vaporize the guts of the AFM.
knunger wrote:My '88 535is idles fine when it's 'cold' outside: <80F.
From 80 to 100F when I depress the clutch pedal approaching a stoplight, the rpm dip to 400-500 for a couple seconds then go back up to the normal 700.
Yesterday, when my OBC showed 121F ambient, the rpm dips but does not recover and the engine stalls. She starts right back up.
WTF??? Any suggestions?
(an aside: How do male Tucsonans tell temp without a thermometer?
Length of the ol' ballsack. When its over 110, we need to wear knee-length shorts! )
Jeez, you wienies down South beat us by one deg F yesterday and things start falling apart.
The first thing I'd do would be to clean out the ICV with some brake cleaner. It may not be able to handle the added load of AC, the fan clutch engaged and the cooling fan running on high.
Another thing that can be temperature sensitive is the in-tank fuel pump. If it's failed you can get vapor lock under some conditions. You'd also notice a hesitation on acceleration.
The 121 was ambient going (slowly) down a 2 lane blacktop. Our official high was 106. You northerners' high was 111 - I'll bet the I10 out by the 202 loop was 130 ambient easy.
I'll bet you guys need to wear 'highwaters' in the summer!
But thanks for the advice: I'm typing right now with greasy fingers. I just cleaned the ICV , have pulled the AFM and am re-reading the forum re: AFM voltage curves.
I had this issue in '12 when I moved to Yuma. It was high 120s, I think. Something. It was hot. The conclusion I came to was that when it's that hot, the main fuel pump swells and starts acting funny. This only happens at low speeds or idle. As soon as you pick up the pace a little, the moving air usually cools the pump enough to keep it from having a fit.
I found that intentionally keeping the rpm up helped at the low speeds/stops.
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