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1985 Jaguar XJS - Cold start problem, help needed

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jaguar85xjs

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Hello, I just purchased the 85 xjs with really low milage (20K) - I'm experiencing a cold start problem, in the morning the car warms nice up but when i place in drive it will bearly go for a few blocks 4-6 - as I pump the accelerator the car starts to react but is hardly driving. Then it wakes up, drives and ultimately performs very well after driving 2 miles. The strange thing is then I park the car for work where it's usually hot - when I start it 8 hours later, and the weather is warm, the car starts right up and immediately performs and basically runs like a new car.

It seems either the cold morning has to be it becuase I don't start it again for 8 hours while at work and it runs great.

I replaced the Throttle Position Sensor and an Idle up control valve attempting to solve - My mechanic is stumped and thinks it has something to do with the ECM or possible bad wiring -We've checked just about everything easy to check. does anyone have thoughts to give me a hand in solving?
 
Is this an EFI car or CIS? I'm not near any reference just now... I kinda recall it's EFI by '85, so: The things are HIGHLY voltage sensitive. Check all your ground points, including the ECM and the fuel rail. Another thing is the delivery pressure at the rail. The front side pressure should be around 28PSI, IIRC. The other suggestion is the "head temp sensor".
 
And WELCOME to the forum!!!
 
hi Bob,

if it's a 6 cylinder, it would be the same engine and system as a Series 3 XJ-6, (1980-1987),

1) check the Cold Start Injector by unscrewing its 2 hex screws, and having someone crank the engine for a few seconds as you hold the injector in one hand facing down, (not to your face), to see if a jet of gasoline spurts out when cranking.
**note that the C.S.I. gasket may tear and you might have to cut a new one to reinstall the CSI and not get a leak, so keep that in mind when pulling it.
If the CSI is not jetting fuel, it makes starting almost impossible, but if you do get it started, stumbling as you describe is one of the symptoms.

2) another possible failure is the Coolant Temperature Sensor located at the intake manifold, those start to go bad causing many symptoms as you describe.

I'm sure more tips will come up here. I really doubt it is the ECU, (Engine Control Unit), those are almost bullet-proof, and rarely fail. For example my 1984 XJ-6 ECU has not given me any problem in 20 years of use and abuse. :smile:)

Ex
 
Hello Bob, welcome, and welcome to XJS ownership.

There are a lot of things for you to get familiar with. Was the car used lightly since 85, or was it left to sit for years? First thing I would check is the temp sensor located on the left hand thermostat housing. If it still has a "brass Hex" to screw it in, replace it no matter how it tests. The current replacements are "plastic coated" and are much more reliable. Second, take a fuel pressure reading at the fuel rail. Disconnect the hose from the RH pressure regulator, jump the relay in the trunk, and take a fuel volume reading also. There should be plenty of fuel there in 30 seconds The exact amount is in the manual. If you do not have the factory manuals, get the CD from JDHT. They were around 50 bucks, and very well worth it. If the volume or pressure tests are low, check the small tank located under the battery. The main tank drains into the small tank, and the pump picks it up from there. There is a filter sock on the pick up, accessed through the hole in the tank (Looks like a fuel guage sender hole, but no wires)
Remove the distributor cap, the rotor and clear plastic shield. Refit the rotor. Move the rotor back and forth. It should move fairly freely in the advance direction, and snap back when released. If not, it has to be freed up. I have had great success usin Kroil. Put a little under the rotor and let it soak a few hours. fit the rotor, and try again. Rinse and repeat as necessary until it frees up. While you are there, check that the vacuum advance actually works. If not, replace it. They are cheap enough, and are needed for proper operation.

The next thing to do is the engine set up. When you replaced the throttle pot, did you track down a black one as fitted originally? Or convert to the common red replacement? Did the pot get set correctly?Should be set to .28 to .32V with key on and throttle closed. Both throttle plates should be set to.002 with a feeler guage at the top of the throttle plates. Then adjust the link rods so that both throttles open exactly at the same time (best done warm, as it might change as it warms up.) Check the bushings for the shafts that actually open the throttle plates. They also go bad, and synchronizing them then is impossible.
This ought to get you started, and I have to go paint an E type header tank...

Good luck,
Jesse
 
a little more information which happened today - the cold start problem happened all morning, the entire way to work the car would do only 25 mph, while floored. I thought it was dead again. when I came out in the afternoon, the weather was very warm (and the car has run well just about every time it's been warm) - So i putted for 20 city blocks, flooring it with almost no performance and going max 25 mph - again, thought the car was dead. Pumping the accellerator, which usually works after a while wasn't working - so I started violently pumping it, nothing much for only 2 blocks. the light turned green, I floored it the way I did for 20+ blocks with no performace and the car exploded forward going from zero to 45 mph in a half of block, it spilled my drink all over me. Car ran like a brand new car off the showroom floor the rest of the way home? does this sound electrical, the ECM, mechanical? I just don't get it, to me it seems like it's possible there's corrosion somewhere when finally heats up and the voltage comes to life at a connection? I could be way off, can anyone help? i really want to keep this car, the way she drove today when it came to life today was like no other car i ever drove, but this morning routine and 20+ blocks prior to that ruins it. bob
 
Well, the car needs fuel, and spark to run. The ECU determines the fueling with sensors. The coolant temp sensor tells the ECU how hot or cold it is, and the ECU adjusts the open time of the injectors to suit. The contacts in the plastic connector can corrode, and sometimes will get pushed out. This needs looking at. The sensor itself needs to be checked.The ones that came in your car were problematic and have been replaced with an updated part.Check the resistance across the 2 terminals. I do not have the manual handy, but the figures are the same for L jetronic (series 3 XJ6) as well as the P system in the S
The other input is the throttle pot. Like I said before, the initial setting is critical. Then the voltage should rise evenly to about 4.5V at WOT. There should be no skips or drop outs in the voltage. The TPS tells the ECU how much you have your foot in it. There is also a small vacuum hose attached to the crossover pipe. This goes to the ECU and and connects to an internal sensor (the manifold pressure sensor) that determines the load on the engine. There do occasionally go bad, and having the ECU rebuilt is the cure. You can find the latest and greatest ECUs used, and just swap one in. They were used through 89 at least.
These things need to be checked with a volt meter, and an ohm meter. The ECU cannot process incorrect inputs, so you need to determine that the inputs are correct. Bringing one of these back from the dead can be a long and daunting task, but in the end they are great cars. Smooth, fast, and like nothing else on the road. Look for the S 57 publications. They had engine managenent, climate control, really good wiring diagrams, and probably others I can't remember.They were given to the dealer mechanics when the cars were new. Look on Ebay, or check the CDs. Look for a service CD that mentions technical guides. These will help a lot!!. Information is power, and with the right info, fixing one of these is not to bad.
 
I had another thought. Has anyone dropped the exhaust and looked at the catylitic converters??If you unbolt the pipes at the rear of the mufflers, and loosen the clamp on the secondary converters, you can , more or less, easily remove the pipe, and look at both the front converters still on the engine, as well as the secondary cats. I have seen a car where the cat substrate broke up, rattled around until it formed a ball, and would roll back and totally plug the exhaust pipe. Very cool, and similar symptoms. No performance at all, until you stopped, and it rolled forward.... and then off it went. Not a likely scenario, but maybe worth a look.
 
Hello,

I wasn't sure if it were a related problem but I just replaced all 4 catalitic converters, so those are new. My mechanic feels it's the ECU giving incorrect signals or processing bad data - or faulty wiring in the web maze of wires. The weird thing is, like today, the car either doesn't drive or is the best ever - which is frustrating - I guess the only thing to do is to pull the ECU and test for failures?
 
The ECUs rarely fail in these cars. In all the years I have worked on them, I might have replaced one. They can fail, just like every other part.The thing is, any 85 to 89 V12 XJS ECU will interchange. ( Probably plenty of other years as well) Just borrow one from another car, and see if the problems remain. The only wiring problem I have ever seen was when some bozo installed an old school phone in the passenger side of the console, and ran the screws through the main harness. You are far more likely to have issues in the engine compartment. Look at the things I have mentioned in my previous posts. Most running problems in XJSs are found there.I have worked on them since the 80s, with a 5 plus year stint at the dealership. You need to find someone like me where you live. Most of them will be in their 50s, and either working for an independant shop, or owning it. They are easy to fix, but you don't want to pay someone to learn the ins and outs of a 25 year old Jaguar.
 
Jesse said:
The coolant temp sensor...
Click to expand...

That's TWO. It's accessible and the symptoms point to it.

I hadn't thought about the dizzy advance issue but it's been a while since I've seen a Jag. I'd be shocked if it were the ECU.

Without the manual or CD for reference you're (or your wrench) are flyin' blind, as well.
 
I posted a cold start problem recently - Well, it was either the Power Resistor Pack or ECM - The PR Pack was pulled, mechanic stated the Pins test should produce 7-10 Ohms and mine was producing 5.35 - So I purchased a used one from Jagbits and when it arived it was testing 5.40 Ohms - Jagbits says it came off a perfectly running car - I then changed the ECM with a rebuild and car fired right up (cold start problem resolved), seems as though the ECM was the problem all along. Mechanic still worried about the 5.40 Ohms reading on the PR and thinks it could eventually corrupt the new ECM.

Any thoughts, he pulled the Ohms reading from a paid site - I thought it strange that mine, and the replacement were both so close - anyone have any literature showing the pins test Ohms for the Power Resister Pack?
 
So... I'm properly shocked. :shrug:
 
yeah, me too.
 
well, I'm sure it was a combo of stuff - after all, in the month I've owned this 85 Jag (which only has 20,500 Miles) I've changed all 4 cats, air valve, air & coolent temp sensors, throttle position sensor, PR, alternator - and after all else fails, change out the ECM - Jagbits loves me right now, didn't know what I was getting myself into to be honest, purchased the car from Ebay in "great running condition" - karma pays him back at some point I believe.

It drove like a brand new car today, it turns heads here in San Franciso - not another on the road in the city, hope she goes for a while. Thanks for your advise.
 
Next time you could save yourself some grief taking it to a reputable Jag mechanic with a diagnostic computer and for a reasonable fee had them tell you what code or codes it is throwing.
 
live and learn, what an ordeal - I didn't realize these cars were so darn specialized - my mechanic owns a 90 Jag himself, super smart / trust worthly guy - Did much of the work I described for free trying to solve - this one caused him much grief too I think - The sensors, which I may or may not have needed when all said and done, weren't cheap but didn't break the bank either.

I think you're right though, best to take it to a spot which exclusively does Jags - For now, I can't wait to drive it tomorrow.
 
The problem with HE V12s is that there is NO code reader, and no codes. It is old school testing all the way, by hand and experience. I think they are easy enough to service, but they appear scary to folks that don't know them. For ease of service in the future, find all the S57 service publications. Jag put out ALL of the info you would ever need, but they do not make it easy to find. The S57 books include really good wiring diagrams (multi page circuit by circuit), the Engine performance manuals, and so on. Well worth the time spent to find them.

Glad the car got fixed in any case. You are right. There is nothing like a good XJS. They are distinctive, fast, and a cruiser par excellance. and the cheapest way to V12 ownership that I know of. The only problem now is that once you have been hooked by a V12, it is really hard to go back.....
 
:lol:

I've helped friends get XJS's on the road. So-far been able to resist th' lure of actual ownership of one.

Have a great tale of top-end fuel fire turning into a CHEAP buy for one.
 
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