86-95 Trucks & 4Runners 2nd/3rd gen pickups, and 1st/2nd gen 4Runners with IFS

Rough idle mystery

Old 12-18-2018, 04:54 AM
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In an effort to keep a good thread rolling... I've had my 91 Toyota for the last 14 years. It's a V6 4WD manual with right at 215k miles on it... For as long as I can remember, it's never idled below 1300 rpm or so and yeah it wastes some gas, but the truck has always run great and never left me stranded. It was a daily driver for year, then became a camp truck for 3 or 4 years... and I recently did a revival on it to make it a run around town, a few days a week driver again. I removed all the dents, did a color change from red to quicksand tan, new lights all around, complete new A/C system, belts, pulleys, fluids, as much as i could, you get the idea... including a new timing belt and water pump... it has zero issue doing 90 mph 3 hours each way on the interstate from my house to my hunting camp. BUT, what it does have an issue with is surging when my foot is on the brake and i'm sitting at a red light or stop sign... even applying the brakes while idling through my work's parking garage.... putting my foot on the brake makes the engine surge up to 1500, cuts and drops to 1100.

What i've tried so far is changing out every vacuum line i can locate. I've blown shop air through the coolant passage of the throttle body, i've replaced the TB intake hose, the small hose from the TB hose to the valve cover. the PCV valve, I checked the timing which was at 14 deg BTDC and set it to 10. I've also sprayed down every inch of this engine assembly with brake clean/ carb cleaner... trying to get the rpm to surge and find a vacuum leak. I've also tried adjusting the stop screws on the TB itselve to adjust the high idle, with no luck. I just saw a video where the guy adjusted something else on the TB, and i'm going to rewatch it at home and maybe do that to address the high idle.

What i'm about to try is changing the fuel filter.. and anything else you guys recommend.

I've been turning wrenches since age 5 and this truck has me scratching my head. Of course trying to find time to effectively work on it with a 1 year old pulling on my leg is also a challenge.


Last edited by bush_hog; 12-18-2018 at 05:01 AM.
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Old 12-18-2018, 07:17 AM
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Have you checked the brake booster? The fact that the idle changes when you apply the brakes suggests you could be pulling air through a leaky booster.

As far as the high idle goes, the idle speed is controlled/adjusted by the idle speed adjusting screw on the throttle body itself. It is NOT controlled by adjusting the throttle linkage or the stops on the throttle plate. I've attached the tune-up section from the FSM. The idle speed adjustment procedure is at the very end of the document.
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Tune-up.pdf (695.1 KB, 90 views)
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