JCF

Hi,
A customer has a problem on his Carrera 3.2. The engines backfires much at idle and when its cold. When hot, it backfires less but engine doesn't run very good.

So we have done the maintenance :
- Valve clearance adjustement
- Renewing of the entire ignition : wires, spark plugs, distributor cap, rotor
- Oil drain, filter, fuel filter, air filter.

I thougt that the old wires were responsable of this problem, but after changing it, backfires are still there.
We try to set the mixture at the air-flow sensor screw. Nothing happened, the CO and CO2 doesn't vary...
The car is not catalysed, and there is a "factory shunt" on the plug for the lambda probe. There is also a plug near the DME under the driver seat.
I will control the sealing of the mixture screw but I don't think the problem is there...
Is it a known problem ? (leak in admission ?)
Is there a plug to control the ignition advance is fitted for Europeen cars, and which "cancel" the no lambda presence ?

Thanks, and excuse me if you don't understand me at all, I'm young (and French )

madmmac

Classic symptoms of intake or vacumn leaks. Check torque and gaskets on intake runners and inspect all hoses.

JCF

I removed the air flow sensor, and cleaned the "by-pass" and controled the sealing of the screw. When removed, I controled visualy air hoses, torque, vacuum to fuel pressure regulator, leaks in idle screw on the throttle lever : nothing seems abnormal.
It's not easy to control all hoses and to find a leaks between the intake pipe and the cylinder head.

One other thing, when starting cold engine, it runs good during approximately 20 seconds, and then backfires.

The L-Jetronic of this car is very simple, and there is not many components inside. I don't understand why it doesn't work.

If somebody has an idea...

Peter Zimmermann

As "mad" suggested, check for intake leaks. While the car idles spray WD 40 or some type of aerosol brake/parts cleaner at the base of the intake manifold where it attaches to the head. Use very small, quick bursts of spray above each cylinder, one at a time, and wait a couple of moments before moving to the next cylinder. If you have bad gaskets the idle speed will increase and the car will run smoother. I've done this on hundreds of cars without a fire, but have a fire extinguisher handy anyway!
Pete

JCF

Thanks for you advices. I think I will try this tomorrow.

JCF

I removed the hose up to the throttle lever, I controled all torque of air hoses, even those behind the big of depression and recycling of the oil vapors, and as adviced, I tried to spray "penetrating oil" near all gaskets between intake pipe and cylinders head, and around the injectors, nothing changes.

I think I will try again with another spay.

I have two wires unplugged near the DME pins, is it for Altitude compensation ? Is it normal that they are no shunt like the plug up to the left head cylinder cover ?

JCF

The leak must be very very small and bad situated, I didn't find it. I surrender.

JCF

In fact, the fact that the car doesn't run good bother me and I try again to find the leak.
The idle didn't stabilize when I spray around the intakes, but when I accelerate a little the engine, I saw big bubbles around them.
I called a part seller and ordered all the seals. I will post when they will be changed.
Still thanking you for your invaluable councils.

madmmac

How did the car run prior to your working on it?

Was it backfiring before?

The 2 wires you are seeing at the DME relay are for the altitude, which should be connected and I think if it is brown is for smog for a California car, which you would leave disconnected.

JCF

I always knew the car running thus, like the owner. It never bother him, and I am not even sure that it noticed his car runs badly.
But he asked me to do the maintenance, and I can't leave a car in this state.

Concerning the altitude correction, are you sure that Europeen car was fitted with it ? The two wires are unplugged...

madmmac

Take a look....does it have a circular gold colored switch?

JCF

This switch would be on the dashboard ? Where ? I don't remember to have seen this switch...

madmmac

The altitude switch is sitting right next to your DME relay by your DME under the front seat.

JCF

Ok, the altitude correction is present a,d plugged.
The brown wires I talk leads from the end of the DME wire harness to two plugs male and female which could be connected together. They were disconnected and insulated, so I let them thus. I thing they are ground wires, but why disconnected ? (I don't have ever test them ).

JCF

I received my parts this morning, and I removed the two intakes.
Two lower seals are in very bad condition as you can see on this picture.
We can also see residues of combustion on the machined surface of the cylinder head where seals were failings.
I will put all this together again at the beginning of the week after after a good cleaning