Hello all -

Background-
I recently purchased a 1994 XJ12 and got her running. Originally replaced both fuel pumps, fuel filter, cleaned fuel tank, blew out lines, cleaned injectors, checked and cleaned connections on all fuses, replaced plugs, and cleaned the distributor cap and rotor, etc. She ran fine for a few days with no missing or hesitation...

Currently-
Now she will crank strong every try but won't to start. I checked for and got spark. Then I disconnected the fuel line at the rail and almost nothing came out when I cranked it. Located the inertia switch and its button is in the down position like it should be.

The big question-
Next is to check the fuel pump relay, but dont know if its located in engine compartment or under the dash. There are 4 relays on the firewall next to the right hood latch that are all identical, but dont know what they are for. I havent looked thoroughly behind the glovebox yet, but taking it apart I still haven't seen any there. Any experienced insight is most appreciated.

Thank you, Wes

 

Also -
It may be the crankshaft position sensor or the flywheel speed sensor... I located both, but havent messed with them yet. Maybe need cleaning anyway because its real dirty under the front of the engine.

She is also having some other minor issues, primarily one that might be related. Sometimes she starts 1st try, sometimes need to turn ignition on and off 3-4 times to 'prime' before she starts. This gradually got worse each day.

Thanks again in advance for any help. Wes

 

Hi Wes,
Again, welcome. A Haynes manual can really help with stuff like this, and they're not that expensive. If you plan on keeping the car long, the full shop manual on CD-Rom may not be a bad thing to have on hand.

Haynes puts the fuel pump relay at the rear of the trunk on the right hand side (behind the trunk carpets) - light blue with a black base. It looks to be a stand alone relay, so it shouldn't be difficult to find. I cannot say, however, what differences may exist between the XJ6 and the XJ12; but I can only imagine this would be the same. I once had a flaky fuel pump relay that caused an intermittent start/non-start problem. Let's hope for that!

A failed Crankshaft sensor is also a primary culprit in non-start situations. To my knowledge, replacement is the only remedy - and it's not exactly a cheap part anymore at $100!
Cheers!
Scott

 

Thanks for the reply Scott...

I have looked for a haynes manual, but all I have seen dont say XJ12 in the title, only XJ6- although inside it may mention the 12 on certain items that are different. The shop manuals on CD rom would definitely be much better, but all I have seen are upwards of $100 or more. That is why I haven't jumped on either yet.

I will tear apart the trunk and will look for the the relay there as you describe- I know some things got moved around year to year. Offhand I dont recall seeing a relay in the trunk when I had it apart for removing the tank, just ECU's etc, but I hadn't thoroughly taken stock of everything in the trunk either. Since they are known to fail with age and without advanced notice I may as well go ahead and replace it anyway. I'll get back to you and confirm if it is indeed there. Seems like that might be one of the many items that are the same between the XJ6 and XJ12, although maybe a 2nd relay for the 2nd pump.

I have found the CPS for as low as $70, but the speed sensor in the bellhousing is around $125 and up. I haven't priced cam sensors yet, but that's further down the list of culprits- the fuel relay is 1st... I just need to figure out which relays and ECU's etc were put where, and all assistance is greatly appreciated.

Thanks again Scott, and thanks in advance to everyone else. Wes

 

That is exatly where it was Scott, thank you- replaced it and she is starting fine every time now. Specifically it is behind the carpet in the r/r corner on the side mounted above the r/r light control box, and way up high so kind of hidden. It was still the light blue Hella relay so most likely the original one.

Wes

 

Hey Wes, check out the Download link in my signature. Off Site/XJ40/ don't have the V12 version but there is a lot of commonality.

 

Update... I spoke too soon- when I replaced the relay it worked fine for about a day, then commenced to refuse to start again... Just today I found the post/thread "fuel pump relay" in a "XJ81" search:

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Original Post: by tipn22 02-16-2012, 02:50 PM
ok so i got the new fuel relay and it still dosen't want to start right way after its warmed up but it does after like 7 trys. i guess the pump isn't kicking on but if i use the wire to cross the prongs it will kick the pump on right away and drive tell i stop and un hook the wire. is there a part that kick relay on and off that i can buy ?? or is there some thing that needs to be cleaned ??

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I have been having this same problem- except mine wasn't even trying to start no matter how many tries, reguardless if warm or cold. I have been "jumping" the prongs in the relay plug for about a week so far just to drive it. The new relay is good, and even tested the old relay and it is good too. The reply for that post is:

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Original Post: by robhair 02-19-2012, 03:36 PM
Try changing the 02 sensor heat relay third one from the left on your firewall and also check gas cap to see if it's sealing off the fuel. I just finished doing both and she starts right up !! just move your ac clutch relay over to heat sensor see if that works before you buy a new one

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I tried it tonight- placed the old (good) fuel relay back in its plug and replaced the O2 sensor heat relay with my new relay. So far its working like it should, but we'll see... The prongs on the old O2 heater relay and its plug were corroded, so I will go back tomorrow and try cleaning the plug as best I can- for now I just wanted to check if it fixed the problem.

BTW, this post also has a pdf for 93-94 relay locations & identification- sorry I dont know how to link it to this thread or I would have.

Thanks to everyone for your help. Wes

 

Update... 18 days later and still working as advertised without lapsing back into "crank-no-start" mode. Seems the O2 sensor heater relay was the culprit on this one.
Thanks, Wes