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Weird Rev hang and rise after Throttle body cleaning

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So because I’m old school and I was doing some prep for my MOT, I thought I’d give the inside of the throttle body a quick clean now that the cars done 45k miles.

 

Now, when driving, and pressing the clutch coming to a stop, the revs will drop to 1krpm but then rise up to 1.5 or 2krpm until I’ve come to an absolute stop. There seems to be a bit of Rev hang and delay when decelerating too.

 

Has anyone cleaned their throttle body and had this issue?

 

I’ve tried removing the EFI fuses and recalibrating the throttle pedal (pressing it 5 times slowly while ignition is on but engine off)

 

 

 

To clean it, I used carb cleaner and a rag and a load of what I assumed was soot/dirt/oil residue came off from around the butterfly and behind it.

 

The actual bore of the throttle body was clean, and so was the front of the valve. But there was a circle of black around the edge of the valve and in the bore behind it. I can only find one post of the FT86 forum in which one person suggests it’s a molybdenum coating to reduce friction and help seal around the valve.

 

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Is the picture attached your throttle? It doesn't look very clean, in fact it looks rather sooty. You may have inadvertently flushed some debris in there, either at the plate interface and/or you've damaged/stripped the bearings.  It does sound like a sticky throttle.

Try a clean and regrease? But not gobs of grease.

These engines are MAF based control and those sensors are easily contaminated. You've not upset it whilst taking the AIS apart have you? A bit of sensor cleaner might be in order.

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Yup, that was the before pic, and that’s what I thought as well.

But every photo I’ve seen since of TBs on these cars all have the same “soot” around the valve. That’s why I’m starting to believe it is actually a molybdenum coating, rather than soot/dirt.

I didn’t actually spray anything into the TB, just carb cleaner on a rag to wipe it all “clean” lol - so there shouldn’t be any debris

I’ve got some CRC maf sensor cleaner on the way, but there’s no issues with idling or stalling, or general running other than mentioned in my first post.

I don’t think the valve is sticking either because as soon as I come to a complete stop, the revs go back down to normal idle. It’s just that period of pressing the clutch in to actually halting where the revs drop to 1k then rise up to 1.5 or 2k rpm.

If the coating on the valve was there deliberately, then it would have reduced the amount of air that can pass thru when no throttle is applied, so now I feel that the ECU may effectively be seeing more air than normal with the valve in that position, so is having to compensate to close it a little further. I dunno, but it’s doing my head in

Will try disconnecting the battery tonight, and if that doesn’t sort it, I’ve read that flashing the Map/tune back onto the ECU can fix it.

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Maybe you've inadvertently knocked a hose and have an small air leak somewhere?

I had similar symptoms (slightly hunting idle as well) and I hadn't tightened down my intake so I was leaking air.

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Idle is spot on. I only removed the intake hose from the TB, and disconnected it from the sound gen tube and airbox. I actually used a ratchet to tighten the clips back up, so I know they’re good.

 

But I’ll be removing the intake hose again to clean the maf now that my maf cleaner spray has turned up. So I’ll double check for leaks, I always forget to check the simple stuff so cheers for the reminder

 

 

I found this pic of a brand new engine on the Perrin website, the TB has the same black stuff around the valve so I’ve ordered some dry moly spray to recoat the valve and rear TB walls.

 

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I don’t wanna speak too soon, but cleaning the MAF and resetting the ecu last night using this method https://www.ft86club.com/forums/showthread.php?t=68328 seems to have improved it.

There still seem to be a little bit of Rev hang changing gear, and deceleration doesn’t seem to be immediate but it hasn’t spiked higher than 1200rpm when dipping the clutch coming to a stop which is the main improvement. I just can’t remember if all that’s normal behaviour or not lol

I spent £20 on some special Moly spray and £30 on a new gasket so I’ll probably remove the TB after the MOT and give it a new coating anyway and hope I don’t make it worse

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