"Tim.." <the.farm.no (AT) spam (DOT) btinternet.com> wrote

Tim, I haven't got around to checking the signal from the MAP sensor, but it
seems the fault has taken a new twist. While I was driving it yesterday it
appeared to be picking up speed by itself and when I depressed the clutch
and took my foot off the accelerator the revs sat at 3000 rpm! It did this
intermittently- sometimes the idle was fine, sometimes it was at around
1000-1200 etc. I managed to drive a good mile without touching the
accelerator at all! The prime candidate for this fault seems to be the IACV,
but I've just changed that. It doesn't rule out the new one being a dud of
course. I'm more inclined to think the IACV is being wrongly controlled due
to the ECU getting duff info from somewhere (TPS, perhaps? I'll recheck it).

My (layman's) theory is the TPS is dodgy and is telling the ECU that the
throttle is open more than it is, causing the ECU to inject more fuel and
therefore richen the mixture, initially causing lumpy idle. The ECU detects
the rich mixture and opens the IACV to allow more air to attempt to weaken
the mixture, causing the high idle. Does this sound feasable to you, Tim?

When I parked up I thought I'd gradually depress the accelerator and see
what happened. The revs didn't pick up smoothly and when it got to around
2500 (IIRC), it hit a spot where it was speeding up/slowing down
rhythmically. Increasing the throttle past that point actually caused the
revs to dip quite a bit, it seemed to struggle then pick up again.

On the road, I've noticed that it now runs out of steam at around 4500 rpm.
IOW, it's not keen to rev anymore, though it performs well up to this point.
Cruising on the motorway at 70mph it seems perfectly normal, no misfiring or
anything like that.

I'm at a loss, do you have any suggestions based on the above info? I'd bung
it in the garage but money is really tight right now and since I used to do
all my maintenance myself, I'm able to change most easily accessible
components. I really appreciate any help you can give.

Morse

"Morse" <morse1966 (AT) morse1966 (DOT) co.uk> wrote

TPS is the usual cause of a stuck high idle, check its output..

tim..

Tim.. wrote:

Tim, I spent a while with it this afternoon. The voltage from the TPS is
0.955 at idle, close to 5V at full throttle. When the revs get to around
2200 rpm, it fails to increase with more throttle, but the voltage from the
TPS increases as one would expect. Keep increasing the throttle and it
reaches a point where the revs suddenly pick up and hit 3000, then all
seems normal.

I also noticed if you snap the throttle open quickly while idling it
hesitates before it picks up. The hesitation makes a slight 'cough' and
there's a very brief dieseling sound before it picks up.

I'm still stumped! I couldn't get it to stick at high revs like it did the
other day so I couldn't take any measurements. I might rig up a
semi-permanent voltmeter in the car so I can monitor it on the road. A faff
on but it might give me some insight as to what's happening.

Morse