Rough Idling When Stopping and Fuel Smell | MK3 Cavalier 1.8i (single-point injection)

[MeanDumpsterCat]

Hello.

My MK3 1.8i Cav has started to develop some idling issues and a fuel smell.

When I come to a stop the revs get very low and then return to normal again and if I'm driving with the windows open there's a very definite fuel smell.

I'm suspecting it might be sensor related but I've not been able to locate the MAP or MAF sensor anywhere. Could anyone provide some pointers or even better some pictures of where these are located on my engine?

I've yet to look at the EGR valve as that also might cause an issue if its blocked. I'll update the post once I get a chance to look.

To avoid any confusion, here's an image of the car I'm looking at.

Any help is really appreciated

[Robsey]

The MAP sensor is mounted under that flimsy black plastic scuttle cover at the far rear of the engine bay.





Looking at your picture - line of sight... follow directly backwards from the distributor.
It will be a little block with a hose on it, clipped to the bulkhead.

[Robsey]

With the fuel smell - are you sure you do not have a fuel hose leak.?

With the introduction of E10 fuel five years ago, it is highly likely that the original fuel pipes will have been eroded, and so may be leaking.

Otherwise, my head struggles to find a drop in revs and fuel smell.
Fuel smell to me is either a leak or running rich.

Idling issues - look at all your tiny vacuum pipes.
Or should I say the rubber elbows. They crack, split or snap with age causing an induction air leak.
There is one on the EGR,


two on the throttle body cover thingy, and
one on the air filter housing.

Perhaps this is weakening the mixture, leading to the Oxy sensor telling the ECU to throw more fuel in.

[Robsey]

When my MAP sensor failed...
The issue I had was the plugs sooting up, making the car very hard to start when warm.
It idled and ran fine, but I didn't dare turn it off unless I was willing to clean the plugs, or wait for the car to cool again.

Okay another thought - the engine coolant temperature sensor..



If the engine is cold it idles fine, but if the engine is warm, but the ecu thinks it is cold, then again it will over fuel and cause a rough idle.

If you are old like me... it will act like a warm engine with a carb, and with the choke on.
If you never had a pre-1992 car with a carb, then you will not have a clue what the hell I am on about.

So there you go...
Vacuum pipes and elbows,
And ECU coolant temperature sensor at the back of the thermostat casting. With a blue [ - - ] shaped two pin plug on it.

[James McGrath]

Could also be worth cleaning out the idle control valve. I don't have any photos to hand, but you can just see it in the photo of your engine about. It's the black thing lust below the top hat and near to the egr valve.

[MeanDumpsterCat]

Thanks for all this. I'll take a look at them.

My plan at the moment is to replace all of the old looking vac hoses as thats cheap and easy.

I unplugged the MAP sensor with it running and the engine sound changed so I think it's working.

I do remember this car had an issue with a sticky throttle which I think was related to the idle control valve. It cleared itself up but it's never been replaced so I'll have a look at that too.

If all that still doesn't work I'll take a look at the EGR valve.

I've never had an issues starting from hot or cold though. It just get worryingly low revs when I come to a stop and then fixes itself.


Would a faulty coolant temp sensor cause the temp gauge to not work? Because that seems to be functioning okay.

[Robsey]

The temperature gauge sensor is on the front of the thermostat housing.
It is a single Lucar (female spade) connector on the 'T' shaped tip of the sender.

It is separate from the ECU temperature sensor at the back of the casting in my photo from yesterday.

So, no they do not affect each other.

The C18NZ single point injection engines is more or less a Rochester Multec throttle body thrown on top of the inlet manifold in place of a carburretor.
It is very basic, and uses a single ECU-controlled injector in place of what would have been the main jet of the Pierburg 2e3 carburretor.

The only weakness on these is a leaky seal or a sticking injector, but these tend to stick in the open dribbly-state leading to excessive fuel consumption.

My money is on an air leak.
Either at the throttle body, or in all those vacuum pipe elbows.

And I am sure you know that the C18NZ does not have a smooth tick-over sound... it will have a regular dip in the exhaust flutter sound, Not smooth and continuous. This is quite normal.

[iangsi]

I replaced the lamda sensor in the exhaust down pipe when my 1.8 was running rich which sorted the problem.

[MeanDumpsterCat]

Alright, this was a simple fix. Turns out the vac hose going to the EGR valve was absolutely cooked.

Quick trip to Halfords for some new vac hose and it's all good now. The idle is back to its usual 'rhythmic' sound as these engines often do and no more fuel smell (or at least not that I can notice).

I wish every problem was this easy to fix!

[Robsey]

I like a nice easy fix.
All tickerty-boo again.

They like to draw you in with a simple fix, and then....
Bang, it throws a real colly-wobble on you with a proper head scratcher.

Enjoy the victory, while it lasts - lol.