Engine loss of power as warming up

[grantedwish]

I have an 1988 XJS-C V12 5.3 that I need an emission test on (CANADA). She failed first try. Passed on idle but had high level of unburned fuel while on speed RPM test. On a test drive she drives well for about 1 minute after she is started but looses power and hesitate's and can even stall after 5 to 10 minutes. I put my foot to the floor but she splutters and reaches 2500 rpm MAX spits and goes to stall. Testing last night noticed the fuel warning light flashing as in time with the hesitation. New filter and have changed pump with other cars pump. Checking for plugged muffler or Catalytic so far no diff. Car has 77,000 km. Read a post of the same prob but author did not post the fix. Hope that I can post the solution.

[kelsham]

I wonder if the over fuelling could be caused by the coolant temperature sensor? Might be worth checking the connectors. I had one fail and it caused white smoke to issue from the tail pipes it was so rich.

Regards Kels.

[steve_m]

My sympathies. A V12 uses enough fuel when it is working properly so the consumption must be pretty startling if it is not actually burning it all; my old Series II LWB XJ12 used to do 9mpg around town.
How much is unleaded petrol in Canada?
Steve

[XjRestorations]

Hi,
She certainly sopunds to be flooding.
On initial start up - will the engine rev above 2500rpm freely? Does the spluttering and refusal to rev only come in once warm?
I would suspect the thermotime switch could be at fault and the car is running cold start all the time.

Kind regards,
Keith

[grantedwish]

Well seems that all is now good! Changed all plugs set gaps per spec and new ignition cables. Found a crack in distributor cap (replaced) all good for about 20 miles then problem showed again but far worse! Changed fuel filter and bingo!!! Got cleaner in the fuel and going for a fare run tonight in hope! Heard Sea Foam is good giving it a try? I had changed filter before but I used the one from the donor car. Going to change again in a week or so think I have some grunge in the tank as the car had sat for 5 years or so. Thank's for the feed back......

[CliveRothwell]

The figures from the test are the only true evidence that you have: it stated unburnt fuel which usually implies fuel mixture being too rich. This ties in nicely with the thermotime switch being faulty. A glance in the mirror whilst it is spluttering will also give a further clue: darker, visible exhaust smoke. Ask the neighbours first as it's easier to do such a check once it is hot on the driveway.

Cheers
Clive

[kelsham]

I believe the thermotime switch is just for fine tuning. It is the coolant temperature switch that decides the main fuel requirement.

Regards Kels.

[grantedwish]

Return fuel too hot A/C off line for the moment. Pick up tank fuel getting quite hot. I am thinking that the fuel is boiling in the rail and fuel is not being delivered thru the injectors properly. Problem sows up after 10 to 20 miles.

[kelsham]

I have run my two XJ-S cars without functioning air conditioning, did not cause problems. What temperatures are you experiencing where you live?

Regards Kels.

[grantedwish]

Got a great pass on the E-Test. Problem turned out to be the fuel pressure regulator. Was putting 50 psi into the rail not 36 psi. Return line is still heating the pick up tank and making the fuel pump drop pressure. Temp today 23c temp gauge reads between cool and normal. Trying some insulation on fuel lines in the engine area.

[grantedwish]

Complete fuel system clean new fuel pump and filter. Nylon sack on fuel pickup was a Quite dirty could have been a large part of the problem, cleaned well. Fixed A/C and all is working as it should, or so I think no more running issues. Outdoor temp has been in the mid 30 Celsius and no over temp issues while driving.