I had posted this on another forum, but my thread got semi-hijacked and then fizzled. Hoping someone here can give me a few ideas as to what's causing my problem so I don't have to resort to the dealer. I really prefer to do my own work whenever I can.

Here are the symptoms. My car starts and runs fine when the engine is cold. But, if it's been warmed up to operating temperature and then shut off for 20-30 minutes, it spins over and fires up quickly, but won't idle. If I keep my foot on the gas for 30 seconds to 1 minute to keep it from dying, it will go ahead and run ok after that. If the car has set for an hour, it will start and idle fine. It's just the initial few moments after the ~half hour shut down that are a problem.

After doing a little research, I've tried the following:

1. Installed a check valve in the fuel supply line upstream of the A-bank (right side, hope I got that right) fuel pressure regulator. No improvement.

2. Checked the thermal vacuum valve by putting it in a pan of hot water on the stove. It clicks at ~180 deg. F, so I think it's operating properly.

3. Checked the coolant temperature sensor. Ohm readings at various engine temperatures are in line with specs. I think this is also working properly.

4. Installed a pressure gauge upstream of the B-bank (left side) regulator and checked pressures under various conditions. Here's a summary of the results:

Cold engine, started the car and ran for about 1 minute – 30 psig
Shut off engine and let sit for 30 minutes – 35 psig
Took a 30mile drive (30 deg. F outside temp), checked pressure with engine running – 30 psig
Shut off engine and let sit for 30 minutes w/ hood down – 40 psig
Tried to start and it wouldn’t idle properly for several minutes (this is the original problem I’m trying to solve).
Let sit for 30 more minutes w/ hood down – 30 psig
Was able to start it up with no problem.

Took another 15 minute drive so I could check rail pressures at 30 minute intervals without the engine restarts this time – 30 psig on returning from the drive
Shut off engine and let sit for 30 minutes w/ hood down – 42 psig
Let sit for 30 more minutes w/ hood down – 32 psig
Let sit for 30 more minutes w/ hood down – 30 psig

I had to return the gauges, so I wasn't able to see if it held pressure over night, but cold starts aren't a problem. It seems strange that I’m experiencing the problem when the rail pressure is at its highest. Any suggestions you can provide are appreciated.

 

Here are a few possibilities (assuming you have Lucas ignition, not Marelli ignition):

The Auxiliary air valve (AAV) may be playing up. This adjusts the amount of air going to the engine when the car is not fully warm. It is an aluminium device at the cabin end of the engine just to the right of the cabin end of the LHS (US driver's side) air filter box. You will see a short rubber pipe connecting this device to the air filter box, and a big bolt on the side of it pointing towards the wing (fender) which, believe it or not, adjusts the idle speed.

I suggest you block off the inlet into the air filter box and then start the car cold and see how it runs without the throttle helping. Then restart if needed and keep it going on the throttle until warm. Then turn the engine off and wait for it to cool to the 'trouble' temperature. Then restart it and see if it still will not go without the throttle's help, and then if you can still keep it going with the throttle again, when part warm. Then get it fully hot and test the idle restart hot and see if it all OK for the hot start as before.

If after all this, the engine's cold start performance (with the AAV air feed pipe blocked) is like the part-warm performance - that is the engine will not run without throttle help, it is highly likely the AAV is the problem. It might even be that speeding up the idle speed a touch to say 750 - 800 rpm aircon on, will fix the problem. Sometimes the AAV does not quite open and close in exact sync with the engine's needs. A touch more idle speed can get it over this patch.

If none of this makes any difference, post again and it is on the the vac tubes the other side of the engine!

Greg

 

Mmmmmm, read it 3 times, a tad slow on the uptake tonight.

I am assunming an early '89 with Lucas ignition???, and not a Marelli car???.

The fuel pressure running and after some sitting at 30psi is deemed OK. The 40+psi when HOT is another OK as far as I can determine, BUT, that is high by any standards, and the injector ON time would pass more fuel than at 30psi, hence overfueling the engine at idle, EXACTLY what you got, so the boot on the gas for a faster "idle" for about 30+ seconds keeps the engine going and the fuel cools due to new fuel arriving, and the pressure drops and all is sweet.

I do not run a RH fuel regulator, it is a "damper" and serves no real purpose in my opinion. THE lh unit is the actual regulator.

I do not run any check valves of any sort, other than what may be in the pump itself.

I do not run a HE fuel regulator on the LH side, it is replaced with a PreHE adjustable unit with NO vac hose, and the engine is sweet at 29psi, and rubbish at 31psi, so at 40 psi you are overfueling.

Mine never held presure too long with the HE vac regulator installed, it bled off within 30 minutes, which of course lowers the rail pressure, mmmmm. Maybe you got a LH regulator sticking, they do that, and there is your issue.

My suggestions are as follows and in NO particular order.

Fuel pump is getting tired, and slow to get a FLOW to cool that fuel rail quicker, not high in my thoughts.

Fuel filter is restricted, giving the flow issue as well, also not too high in the brain.

The throttle bodies are "gooed up" and that will reek HAVOC with hot idle, clean them with a solvent soaked rag.

The throttle discs are set too close, not the 0.002" required for stable idle.

The throttle cross rods may not be synchronised "spot on", or too tight, and that can alter the discs when things heat up.

The TPS is having a moment, old age is creeping in now, and if that is feeding erratic signals to the ECU, the fueling will be errratic also.

Mine goo up those discs regularly, and I know instantly, so a quick 30 second wipe out, and nothing else, and a SMOOTH engine returns.

 

Thanks for the input Greg and Grant. I'll respond to your questions and comments in order.

Greg,
I don't think it's the AAV. Last winter I replaced cam cover gaskets and did a valve adjustment, and as part of that exercise I had to monkey with the AAV. On a cold start the idle was too high and the ECU would momentarily cut off fuel. This caused the idle to pogo between ~2000 and ~500 rpm until it warmed up. I took the AAV off, cleaned it up and pressed the top in with a vice (per Kirby Palm's book) until cold idle speed was at a reasonable level. now it idles at ~1200 when first started cold and slowly settles to a little over 500 rpm as it warms up. My AAV seems to be operating correctly.

While I was doing the valves, I also replaced all my fuel lines and every vacuum line I could get my hands on. If it's rubber and under the hood, it's probably new. But, while I was removing the old vacuum lines, one of the connections on the vacuum dump valve (EAC4069 - right front of the engine) broke off. This item was hard to find and expensive when I did find a source. In researching the issue, I found that several people had just left theirs off. So, I did the same. Any chance this is part of the problem?

Grant,
I've included your questions below in italics with my responses following:

I am assunming an early '89 with Lucas ignition?
Correct

Mine never held presure too long with the HE vac regulator installed, it bled off within 30 minutes, which of course lowers the rail pressure, mmmmm. Maybe you got a LH regulator sticking, they do that, and there is your issue.
But it starts fine cold. After sitting overnight, there would be no pressure on the rail and this should be the worst case. The problem only occurs after it's been shut off for around half an hour.

Fuel pump is getting tired, and slow to get a FLOW to cool that fuel rail quicker, not high in my thoughts.
It's a possibility, but not high in my thought either.

Fuel filter is restricted, giving the flow issue as well, also not too high in the brain.
Fuel filter and screen in the fuel sump are both new.

The throttle bodies are "gooed up" and that will reek HAVOC with hot idle, clean them with a solvent soaked rag.
These were cleaned last winter during the valve work, but it wouldn't hurt to hit them again. I'll try that next chance I get.

The throttle discs are set too close, not the 0.002" required for stable idle.
I adjusted these during the valve work as well. Will double check.

The throttle cross rods may not be synchronised "spot on", or too tight, and that can alter the discs when things heat up.
Same as the previous answer.

The TPS is having a moment, old age is creeping in now, and if that is feeding erratic signals to the ECU, the fueling will be erratic also.
Another possibility. Do you know of a way to check it out?

Thanks again for your help, fellas. Much appreciated!

 

Not sure what I did, but my reply was posted twice. Sorry.

 

The throttle potentiometer, (TPS) is set up as follows:
1. Switch on ignition, without starting engine, and using a voltmeter check that the signal output on the green wire is 0.32 volts (+ or - 0.02 volts) with the capstan wheel in the closed throttle position. Adjust as necessary via the 2 clamping screws.

The wire colours do vary. There should always be a 5 Volt input on one wire, ignition on, and a variable output on one of the others, and nothing ever on the third. If it is set at 0.32v throttle closed, then it is correctly set. These devices do get erratic though, so a new one is a good idea IMO, though a bt pricey. Rockauto have them.

If all else fails, there is a test you could do to see if the vac tubes and all associated junk are the root of the problem. First though, have you checked that the vac capsule on the distributor is OK? If not, suck through the vac line joined to it and see if you can pull and sustain a vacuum. If it is toast you will just suck air. If so, replace it, will give hints if required.

Anyway, the possible test: Put a vac line directly from the vac capsule to RHS inlet manifold. there is a port underneath the throttle body that is suitable. Disconnect and block off all other ignitionand cruise control related vac lines from the RHS manifold, note them carefully so you can put them back.

Next, on the RHS water rail, that runs below the RHS inlet manifold, you will see at the cabin end, a device with two wires going to it. This is a water temperature switch which, if open, disables the 15 minute timer that does something or other to the system that makes it run a bit richer when the car is started cold. It is explained in "the book". Disconnecting the wires from the switch disables the timer so the richer condition does not happen. The engine now has (i) the vac capsule controlled directly by the inlet manifold vacuum with no intervening bits of hardware, and (ii) the timer on startup disconnected.

Now try the cold, warm, hot tests and see what happens. FWIW, I have run the vac capsule directly in this way, wih the timer disconnected, for years, with not unpleasant effects whatsoever, and a much simpler engine bay.

 

If you have no stable idle, it is not lack of vacuum, but the excess of it. Our cars are extremely primitive, when it comes to theoretical design of injection system, while being very complicated on how it is implemented.
I'd say re-visit AAV valve in general, but also do the following:

when car is hot and does not want to idle, unhook one of vacuum lines and generate small vacuum leak. See if you get your idle back.
Also, take driver side air cleaner housing and filer off and listen to the air whistle through AAV passage at idle, when car is running well, do the same and compare air flow sound when idle goes unstable.

These AAV's are going bad (wax bulbs inside) and no longer available, but usually they get stuck open, so idle is high.
I was able to rebuild mine with the use of Volvo Marine thermostat.

 

Also, you did check the PCV valve? When clogged, or passage obstructed, it will kill your idle.

 

I checked the vacuum advance capsule this weekend and it's shot. It was like sucking on a straw, no resistance at all. I'm not convinced this is causing my hot start issue, but it's definitely a problem. So, I've got a new one on order. I'll fix this and then go back and try some of the other suggestions.

BTW, I had already replumbed the vacuum advance line on the right side as you had suggested when I did the valves last year.

Thanks again for the input!

 

@twerth

I've been following this thread as I have exactly the same problem on my 92 V12, which obviously has a lot of different parts compared to your 89, but my symptoms are identical to yours, ie there's a small time window when the thing will not start, with the only way to get it to start is to hold your foot on the throttle until it gets over its coughing and spluttering stage, then it's perfect, smooth as silk, stable idle.

I also have a permanent fuel pressure gauge attached to one of the flexi hoses on the fuel rails and I have around 40psi of pressure when the car is running. Your fuel pressure readings seem to be your best signal of your problem....... higher pressure (for you around 40 ~ 42) means no start. Weirdly the 92 V12 has a "hot start" system which INCREASES fuel rail pressure for a hot start (I can see around 60psi on the gauge before start up if the engine is hot)

My problems started after I got my A/C fixed and running again. Since I did this, the car tends to run a bit hotter as the compressor is now under load. I am leaning towards fuel vaporization or vapor lock (as my engine bay temps are now higher than before). Have you tried the key on-off-on-off routine to give the fuel pump more of a run before you try and start it when the car is in its "non-starting" time window ?

My next plan of attack is to fit a temporary extra circuit for the fuel pump so I can run it for ~30 seconds before I try a hot start to see if that makes a difference (30 secs should be plenty long enough to get cooler fuel up from the tank onto the rail for a successful start)

Will be interested to see what happens next for you..... will keep checking this thread

 

SARC,
I'll be replacing my vacuum advance this weekend, and once that's done, I'm going to check the coils. I got some information from Grant Francis that suggests bad coils could be the cause. I'll update the thread with the results (fingers crossed).

I have tried cycling the key on and off a couple of times prior to a hot start and never noticed much if any improvement. This seems to support the electrical rather than fuel system as a culprit.

 

Do you have both hoses on the thermal vacuum valve, EAC5086? EAC 5086 EAC5086 Jaguar Vacuum Thermal Valve. Your pressure reading showing fuel pressure rising just after sitting with hood closed supports the vapor lock idea, does your AC work? If not that might aggravate the hot start situation since it is used to cool the fuel before it returns to the tank. A lot of times this valve gets broken and never replaced, the plastic becomes brittle and the hose nipples break off, so you could simulate it's function, our 1989 Marelli convertible came to us with the manifold vacuum just connected to the left or B bank FPR. I have since repaired the valve and have not had any hot start problems.

I have also heard of coils failing when hot, but that's an easy test with a spark tester or just put a nail or screwdriver shaft into a spark plug boot and near ground to see if you have spark during the hard start times. But no spark doesn't have to be coil either...

Another possible hot start failure is the Lucas ignition module on the left-hand intake manifold, inside the Lucas box is a GM ignition module for the distributor, so buy the GM module for cheap not the whole assembly for $$$, I believe it requires heat sink compound when mounting so it obviously gets hot, and I've read more than a few people carry a spare one around in the trunk.

Ron
1989 XJS conv, Marelli
1979 S2 XJ6 FI

 

Ron,
My TVV is connected and I believe it's working correctly. I took it off and put it in a pan of hot water on the stove and I could hear it click when the temperature reached ~180F.

I'm going to check the coils in the next day or two. Grant provided me with a method to check resistance to determine the condition of the coils. This should be quick and easy, so hopefully I'll have something to report soon.

The ignition module is a new one to me and will probably be the next thing I look at if the coils check out. Thanks for your help!

 

This might be out of left field but have you thought that the fuel might be vaporizing in the rail/hoses.

I do not know about the earlier cars but the Marelli has a hot start function to overcome this by increasing the fuel pressure if the fuel temp is over 70°C for 45sec.

Might be worth checking fuel temp.

 

@warrjon

I'm pretty convinced fuel vaporization is my problem. My 92 car does not have the fuel cooler routed through the A/C system, instead it has the hot start system as described above. Mine is working for sure as I get >60psi on a hot start, and only around 40psi on a cold start. My starting can be influenced by the key off on off on trick, so I'm going to do the permanent fuel pump run, which I can connect to my keyless entry, so it will run the fuel pump for 1 minute once I unlock the car.

I was also leaning towards some kind of electrical component (ignition amplifiers for example) getting tired and struggling when it got hot, but then once the car has got over its initial coughing and spluttering, it will sit and idle perfectly, even as underhood temperatures continue to rise, so I'm not sure if this could be a failure mechanism. I have replaced everything in my ignition system except for the ignition modules that sit on the top radiator rail and I am currently trying to source reasonably priced replacements for these.

Unfortunately I have not been working on the XJS recently because I swapped out my '00 XJR for a '06 XJ Super V8 Portfolio, so I have been too busy polishing it, but I'll get bored and get back to working on a "real" Jaguar soon :-)

@twerth

I think someone else asked the question, but is your A/C working ? I had read somewhere else that if the fuel cooler is not working because your A/C is not working or charged, this could lead to starting problems..... ?

 

I drove across Australia 18months ago without AC and had no starting issues even in +50°C temps. How is the engine temp?

I am rebuilding my car and I am planning on removing the fuel cooler and using Fire Sleeves on fuel hose and wiring under the bonnet

Firesleeve Hose from Aircraft Spruce

 

Temp gauge comes to just before half way (12 o'clock) on the gauge at its hottest, just before the electric fan comes on.

Even if my car started in 50deg heat, I doubt if I could drive it without A/C :-) Were you using beer for your coolant ?!

 

The Fuel Cooler does not cool the fuel going to the engine - it cools the fuel going back to the tank. There is nothing I could find in the ROM to indicate where it is in the scheme of things.

Looking at my car the cooler is on the return line back to the tank - I can only think that it cools the fuel retuning to the tank to prevent evaporation from hot fuel.

 

I checked my coils out tonight, and they appear to be ok. I may check again with a hot engine to see if that makes a difference, but I'm waiting for parts right now and can't start it.

Ron, could you give me a little more info on the ignition module? Is there a way to test it, or do you just replace it and see if it helps? Is there a GM part number? Maybe this will be obvious once I take the box apart.

BTW, since others have asked, my a/c works and the fuel cooler gets cold.

 

Has anyone checked the crank position sensor? I had one of those cars leave me stranded two miles from work one time. Sensor finally got hot and went to an open circuit. I think it is more common on the six cylinder engines but it happened to me in a V12. The crank sensor is mounted at the bottom of the crank pulley.