Hi All - My 1994 XJS just recently developed this issue. It starts and idles fine but when I go into drive and press on the gas pedal it putters and won't reach above 10 MPH. A mechanic friend is trying to help but he is stumped. His diagnostic machine is for newer cars so no help. He thought it may be Air Flow Sensor. I found one on eBay and he just installed it but no change. Any ideas? Many Thanks!

 

Is this a 4 litre car? And is the VIN earlier than 194774?

Paul

 

Yes it is a 6 cyl 4.0 engine.

 

By the way VIN ends with 191154

 

Un-plug the maf completely, this will put the car in fail safe and should be somewhat driveable. Also change the fuel filter and check the fuel pressure with a gauge to ensure the fuel pump is delivering enough pressure.

 

I can only assume your mechanic has ruled out a vacuum leak. Still, you can test this yourself by placing some plastic Reynolds Wrap over one end of the MAF and wrap the intake manifold on the other side. Pull the hose off the intake elbow (or take it from the camcover) and blow some smoke in there. You'll need a cigar or cigarette, no vape! If you see smoke, there's your problem. People do make mistakes when reinstalling some of these rubber connections between pipes along the intake. They also get holes from time to time.

Next, check the Coolant Temp Sensor. $20 and 5 minutes of work.

After that, since you've already swapped out the MAF, check the oxygen sensors? When were they last replaced? If you don't know the answer, it's probably time, they are not meant to last forever.

After that we start to look at plugs, checking for oil in the plug wells, wires from the distributor, etc.

Is it idling well, and only stalls out when you give it some gas? Then it could be a dying fuel pump ($$$) or maybe just the fuel filter needs to be changed?