Either you have a vacuum leak, or your throttle body is dirty, or indeed the
MAF is giving varying signals.
The MAF is located just on top of the air box on the hose to the intake
manifold. A quick check is to disconnect it and go for a drive. The MIL will
be lit, but if the idle quality is better or the same (not worse) then the
MAF is at fault.
Check for intake leaks very carefully first though.- espcially the PCV
I would also blank off the EGR valve for trial purposes too. Disconnecting
the actuation will not stop a physically leaky valve leaking.
The only thing that I can see on the intake system which could be the MAF is
located on the end of the bottom of the manifold before it branches out-
Is that the crankcase ventilation system, the small hose which goes to the
airbox? It seems secure but I'll recheck.
I'm not 'absolutely' sure I know which bit is the EGR valve, my guess is
If I'm right, I need to remove the large pipe with the red/black connector
and plug both the valve and pipe?
Your help is much appreciated Tim.
You have a MAP equiped 1.6, rather than MAF- this system works on air
pressure rather than volume. You will need to remove the MAP sensor, as the
small air pipe often blocks with oily crud / deposits. Do NOT use carb
cleaner / air line directly into the map sensor, if its little pipe is
blocked, blow across the top of it with your cleaner.
There is no EGR on your car- the device you marked is the fuel pressure
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.
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
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.
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