Getting some time to nurse my 89 XJ6 back to action again.. Finally got the car running from a recent rebuild and sorting through some suspension rattles, but had an acceleration issue and was curious if there were any thoughts out there on what direction I might start poking around?


Car has been running fairly well, but a couple times recently, I've noticed that once it warms up, I get into a situation where I have very little acceleration and struggle to hit 30mph or so. It doesn't sound rough, or have trouble idling, but just seems like I've got a lawnmower engine in there and it's doing all it can to move the car along.


I have a new fuel pump and filter, cleaned and tested the injectors at a local fuel injection shop...but should I be poking around and trying to test temp sensors, the TPS or CPS, etc..? Any info is appreciated...


Thanks,
Pete

 

Pete,

The first thing that comes to mind is the Coolant Temperature Sensor (CTS). If the sensor is not operating properly, or its wiring harness or electrical connector has been damaged (a very common problem), the Engine Control Module (ECM) will continue to apply startup fuel enrichment, causing an overrich condition once the engine is warm. The CTS is the 2-conductor sensor on top of the thermostat housing. It's a common and inexpensive part, so it's worth replacing just to rule it out. Note that the 1-conductor sender on the thermostat housing drives the temperature gauge in the instrument cluster.

You can test the CTS with an ohmmeter, consulting the table in the manual that gives the resistance vs. temperature. Measure the sensor with the engine cold and again with the engine hot, to see if the resistance is changing correctly. You can perform the same test at the ECM connector to test the integrity of the CTS wiring harness. Honestly, the part is so cheap, and if yours is original, it's so old, that it would be a good idea to just replace it and see if things improve (that is, if you don't see any obvious problems with the wiring harness or connector).

For the record, I had trouble with a Lucas CTS that had a combined plastic electrical connector and hexagonal "nut" for tightening the sender. The plastic "nut"/connector broke loose from the sensor mechanism as I was trying to snug it up and it leaked because I couldn't get it tight enough. Even though the ebay photo had shown a large brass hex for a wrench, instead of the plastic hex on the sender I actually received, the ebay seller, stevelewissaab from the U.K., was unwilling to replace the defective sensor, and to this day he is still listing the part on ebay with the incorrect photo showing the brass hex, even though he admits that he can only provide sensors with the plastic hex!

I spent a little more money on a Bosch sender with a large brass hex and it has worked great. Note that other manufacturers offer the plastic hex style sensors (Airtex, Standard); I would avoid those and make sure your vendor is going to ship you one with the large brass hex.

If the electrical connector for the CTS has broken, it can be replaced fairly easily. It is a Bosch EV1 connector, identical to those used on fuel injectors, O2 sensors, the carbon canister purge valve, and other parts. You can get replacement EV1 connectors from fuel injector shops, racing shops, etc., and you can get them either with pigtail wires already attached that you spice to your existing wires, or with terminals that you crimp onto your wires. In the U.S., a great source for EV1 connectors is DIY Autotune (MegaSquirt | Engine Management System | Electronic Fuel Injection - DIYAutoTune.com), with whom I have no affiliation, but who has provided me with excellent service and competitive prices.

The other thing that comes to mind with rough running after warmup is the O2 sensor, which should flag a DTC if it is out of range, lazy, or failing to swing high and low, so check your VCM for any stored codes that may not have triggered a check engine light.

Let us know what you find.

Cheers,

Don

 

Thanks for the details Don, I'll do some more checking and check back in with any results I need to get a little more road time with it, but did notice I had some tough time starting the car this afternoon after letting it warm up on a hot day in my driveway ...car just wouldn't start up. I had run it for about 10 minutes, then turned it off and back on and wouldn't start....I waited 10 minutes, and it started right up. Fun times


Pete

 

Hi Pete,

The symptoms you describe could suggest that your Fuel Pressure Regulator (FPR) and/or fuel check valve have failed and are not holding proper fuel pressure in the fuel rail (roughly +/- 40 psi). If the pressurized fuel in the rail boils due to a sudden loss of pressure when the fuel pump is shut off, the vaporized fuel in the rail creates classic vapor lock, and the engine won't start again until the fuel condenses back to liquid and can be repressurized by the fuel pump. The photo album at the link below provides some info about replacing the FPR:

Welcome to Jag-lovers - Members Photo Viewing Page

The FPR could also be contributing to your warm acceleration problem. If the diaphragm in the FPR fails, raw unmetered fuel can be inhaled into the intake manifold via the FPR vacuum line fitting, causing overrich running, which may or may not be serious enough to trigger a DTC. You can check the FPR for diaphragm failure by pulling its vacuum hose off and checking for the presence of wet fuel at the fitting (you may need to crank the engine to see any fuel, but do this only while the engine is cold).

IIRC the fuel check valve on your '89 is in the fuel line next to the fuel filter. It's job is to work with the FPR to maintain fuel pressure in the rail by preventing fuel pressure from leaking back into the fuel tank. I don't know if replacements are still available from Jaguar, but it's easy to add an additional inexpensive check valve to the fuel line that feeds the fuel rail in the engine bay. See the photo album at the link below:

Welcome to Jag-lovers - Members Photo Viewing Page


If the FPR and check valve in your '89 are original, then it would be well worth replacing them. FPRs are available for as little as $40 on ebay, and I used a check valve from McMaster-Carr that was less than $30 including the appropriate hose barbs.

Cheers,

Don