172 Cup High and Higher Idle

[Flipmedia]

Having had the head changed on my 172 Cup following it dropping a valve in cyclinder 1, she was running sweet for around 1000 miles.

The issue started with the idle becoming elevated and uneven. Where as it would sit at a round 850-950 once warm, it started rising up to 1110-1200 waverring. This has got progressively worse and was sitting at 2800-3000 standing.

From reading as much as I can on here and a nice guy called Chris... I have been pointed in the direction of an air leak (as opposed to the throttle body / throttle position type eletrical niggle - having chnaged throttle body and peddle). So... I have already gone ahead a purchased manifold seals and a throttle body gasket as well as Brake Servo Vacuum Pipe Tube.

The reason I am posting is I have just spent a bit more time with the car running (idle at 2800) and spraying a little lighter gas around joins, gaskets, sensor mounts etc and areas where an air leak could develop.

After about 4mins of spraying gas here and there, I sprayed a little gas down between manifold in-take 1 and 2 as I can see a sikaflex seal between head and manifold. The idle started to increase massively up to about 4500 with no sign of stopping, until I turned off the engine.

Bingo! This seems to prove the air leak between head and manifold, but now having left a good amount of time for the gas to disperse when I start it up the idle just shoots up until I loose my nerve and cut the engine - at around 5500.

So I am now thinking, the increase in rpm to 4500+ is nothing to do with the gas I sprayed to do with another issue or the spray disloged something and more air is getting in. Could do with a few pointers on drawing a bit more of a conclusive conclusion...

Thanks in advance.

 

[Flipmedia]

Thought I would just check it again and film the result of adding gas...
The incresase in rpm is gone and after a few more sprays of gas, the idle settled down to 1000. There seems to be a lag on pushing the throttle.
Really confused now, nothing conclusive... still sitting between throttle body failing and airleak.
If anyone is interested I'll post the video.

 

[bearne1]

Mine has had similar symptoms this week so post the video and let us know if you get it sorted.

 

[MicKPM]

Okay, so the first problem you have identified is that someone has used something other than a gasket to seal the manifold to cylinder head joint... This needs rectifying either way.

Are there any other bodges like this?

 

[Flipmedia]

Hi MicKPM

There is a Seal between Head and manifold as I can feel the tab at the base of the manifold inlet 4, but yes pretty unhappy with the fact that there is clearly sikaflex applied somewhere in the joint. Worset case is the seal was broke and sika applied to make good, best it was added for extra measure. At this point I think I have to strip it and replace the seal to know.

I should also say that before I the head being replaced I was chasing a throttle position issue, I had chnaged the throttle peddle (Ebay Second so queastionable) and throttle body (completely reconditioned). The peddle was calibrated... I can't say that these changes ever fixed the issue...

I have since took the car for a gental drive around the block, initially drove okay, then of course the issue returned. First thing I noticed was breaking had a lot of vibration feedback, but this likely due to her pulling even with the gas off - so breaking against drive. Quite quickly limp mode came on and the car pulled all time with foot off gas. Idle is then wavering between 2500-3000 rythmically. An engine fault code was recorded, one fault: P0120

I just started her this morning and idle is around 2000, steady!?!?!? At the point all I can say is the idle is varible, undrivable as pulling hard even with gas off causing throttle position error and limp mode. Starting to think throttle body, maybe sticking open or was stuck open hence high idle, maybe even two issues, this plus airleak. The idle soring after gas and then remaining there was odd.

A videos to follow, I've not managed to catch the real strange or extreme behavior descibed above but at least you can here it running.

Cheers Laurie

 

[MicKPM]

A "normal" cold start idle will be somewhere around 1100-1250 rpm which will settle to 850-900ish after a minute. If you have 2krpm, it is still too high and will get higher when warmer.

You'll need to check the entire inlet track I'm afraid. If you have access to a smoke machine it will speed up the process but they're not cheap... Personally, pull it all to pieces and inspect gaskets, mating faces and condition of manifolds.

 

[Flipmedia]

Hi Guys, any input on this would be great!

I have stripped the in-take manifold right back to the head and refreshed all seals form new, cleaned everything and correctly torqued and put back. Followingthis the idle is full time 4000k...

I got the car to a garage with a smoke machine and left it with them to find an air leak. There is no air leak, none found!

During the investigation they removed the MAP sensor and found that the idle would return to normal if you covered the hole... SO they replaced the sensor and fitted a new one - no difference. Ensured the sensor fitting was sealed with ptfe tape - no difference. Remove sensor and cover hole with finger, returns to normal.

I am thinking 4000 rpm idle is too high for an air leak that you can't hear and of course it's been checked over with a smoke machine...

Any smart ideas, I'm back to thinking it is electrical. I noticed that the throttle body is sitting open a crack most of the time, if that means anything...

Thanks in advance for any help, I am loosing my mind...

 

[MicKPM]

Well you've proven its not electrical and is infact a leak by covering the MAP sensor hole, returning the idle to normal condition.

Have you checked the integrity of the breather, brake vacuum and the rubber hose to the Bi-mode control valve?

 

[Flipmedia]

Have you checked the integrity of the breather, brake vacuum and the rubber hose to the Bi-mode control valve?

Hmm, assuming smoke test at this point didn't cast it's net wide enough then.

The car has a k&n air filter the original air box is gone before I owned it (wanting to correct this at a later date). On the manifold there are two blanked hoses, one at the rear next to the brake vacuum connection and a second, a thin hose one at the front just after the throttle body. When I was first looking at this problem I found the hose connected to the front was split and taped up, did a quick fix cutting out the spilt and putting in a small metal tube (this made no difference to the issue). Later I traced the hose back and found that the unit it was connected to (inside the wing next to the horn) was completely disconnected, plug connected to 4 wires was disconnected, looks like it has been for a while, I took this to be as a result of the now removed original air box set up. I disconnected the hose from the unit shortened it to 100mm and wound a bolt into it. This was before the manifold was stripped and made zero difference. Now I am starting to wonder...

 

[MicKPM]

How long did they leave it running the smoke test for?

I'd start simple and remove/block all breather take-off's as it rules out external accessory items such as brake servos, purge and control valves, etc. Also, keep a tin of brake cleaner or deodorant handy (Lynx works well) and spray it at every joint, take-off and sensor.

Mick

 

[Flipmedia]

Cheers Mick... I'll give this a crack!

 

[Lewis24]

Appologies for commenting on an old thread but I’m having the exact same problem with a 182 ive just bought, it came as a bare shell with engine it seems to be a 172 engine I’ve out all my loom from another 182 I have so know it all works but since putting the car together it idles at 4000rpm and still rises every now and then, I can rev past it but the revs won’t drop below, following on I unplugged my map sensor with which the car wants to die every time I let off the throttle and the revs drop it’s also sounds as if it’s back firing when the revs drop. Thanks