Hello everyone, I decided recently to pull my 1988 Vanden Plas out of storage and drive it again, I drove it for a week flawlessly other than a high idle which I rectified a few days ago. However, leaving work last night the car would not start, it would crank but did not have an injector pulse or spark. The next thing I attempted was to read the resistance on the CPS, which read at approximately 1400 ohms. After reconnecting the sensor the car would start, however it would run incredibly rough and backfire through the intake with any throttle applied. I called a friend to go get a truck and trailer, and when we got back I decided to try again to start the car, on this effort it started and ran flawlessly, and did so again once I started it to remove it from the trailer. Is it possible for a crankshaft sensor to fail intermittently? Has anyone had a similar problem to this?

Any thoughts are much appreciated
Evan

 

Hi Evan,

The CPS is known to fail intermittently and to cause all manner of hard-to-diagnose gremlins. An easy test for the CPS is to watch the tachometer/rev counter while cranking the engine. If you see about 200 rpm indicated while cranking, the CPS is probably providing a plausible signal. If the tach reads 0 rpm, suspect the CPS, its electrical connector or harness.

However, in your case, I'm wondering if you may have gotten some fuel contaminated with water, or if after taking the car out of storage it has sat out in the rain. If the drain in the rubber gaiter around the fuel filler neck is clogged with seeds and leaves, rainwater can pool around the fuel filler neck and make its way past the gas cap seal and into the fuel tank. If you suspect this may have occurred, or if you had recently purchased fuel prior to the malfunction, try a couple of bottles of Heet or other gasoline "drying" agent.

Let us know how it goes.

Cheers,

Don

 

Well, after more diagnosis, I'm going to have to deal with my battery first, it seems to have gone bad sitting. Could that theoretically be my issue since it shuts my entire interior down while cranking? Perhaps it's starving the ECU for power in favor of the starter and causing CPS like symptoms? After sitting on the charger all night outside the car it had 10.6 volts and less than an amp. Will update after I get a new battery for her.

 

If the voltage falls below 10.5 volts while cranking the Engine Control Module (ECM) will not trigger the ignition to fire, so you'll have no spark. A very common issue on Jaguars.

Please keep us informed.

Cheers,

Don

 

Thank you for your advice Don B, the battery helped the situation, so far as the car would start, idle for approximately 30 seconds, then break up and die. I decided to retest resistance on the crank position sensor, and found that it would hold 1400 ohms for approximately that time, and then drop, lord knows why. I attempted to source a proper early crank sensor, but the price was rather offensive, however I found that the later XJ40 sensor would accomplish the same job for a quarter of the price, so for 100 dollars and 15 minutes with the soldering iron and heat shrink I'm back on the road and she certainly runs as I remembered from 4 years ago. I honestly regret going to the BMWs for a phase after driving this car again.

 

Thanks for the update Mr. Hotrod, glad you're up and running again. I spend time in Anderson several times a year and have a relative in Muncie who I occasionally get to visit, so I'll watch for your Jags on the road!

Cheers,

Don

 

Hi Mr Hotrod, glad you got it fixed. What years sensor did you use?

 

Hi Jerry, I used a sensor for a 91-94 XJ6