Car: 1988 XJS coupe; 55k miles, Canadian spec.

Problem: Warm restart cranking. Starts very well cold and hot after sitting 5 or so minutes. After that 5 minute period, for anywhere up to a couple of hours it struggles to restart. Consistent.

What’s been done (with no success):

Fuel system: new pump, filter, B regulator. Injectors cleaned and matched, checked them and they are not leaking. Fuel pressure good under all conditions. Bypassed A regulator with no results. Not a problem with vapor lock as I put a jumper switch on the fuel pump and would run the pump for up to a minute to flush the system with cool fuel before trying restart, still problematic on warm restart. Pump does run for a couple seconds upon turning on the ignition.

Electrical: new plugs, wires, rotor, cap. Tried (3) GM modules (inside Lucas unit), removed capacitor in Lucas unit. Cleaned connections on Injector module. Timing correct. Put small fan on cooling line to distributor cap to make sure the internal module was not overheating.

Vacuum; no leaks detectable. all switches etc. are working correctly. Tried bypassing various components to no avail. Good signal to ECM in trunk. Vacuum advance not leaking.
There are other things i have tried that i probably am missing, but any suggestions would be appreciated.

 

Just now leaving for work but one quick thought.

Weak/failing ignition coil? Sometimes coils give trouble after absorbing engine heat. I would more likely suspect such a thing after a long drive...but you never know.

More later

Cheers
DD

 

I had a new coil on it, went back to original 2 coil setup with no change. Back to single coil. Thanks, one of the things I forgot. Also had AAV rebuilt.

 

Plugs gapped at .025"?
Describe what happens when you try to restart. Runs on a few cylinders? Doesn't fire at all? Occasionally fires, but won't keep running?
Could be a hot soak issue with a coil as suggested. Do you still have twin coils, or have you updated to the single, solid-state coil?
I'd find someone with a scope and look at the primary and secondary ignition patterns. I'd also look at the injector signals on a scope. Dirty connections on the injector resistor pack?

 

[QUOTE=jal1234;2477099]Plugs gapped at .025"?
Describe what happens when you try to restart. Runs on a few cylinders? Doesn't fire at all? Occasionally fires, but won't keep running?
Could be a hot soak issue with a coil as suggested. Do you still have twin coils, or have you updated to the single, solid-state coil?
I'd find someone with a scope and look at the primary and secondary ignition patterns. I'd also look at the injector signals on a scope. Dirty connections on the injector resistor pack?[/QUO

 

Gap is .025. Engine just doesn't fire. When it fires it runs great, quite smooth and even. I would agree on some sort of hot soak issue, but the problem exists after a couple hours of sitting, with bonnet open and Everything is cool to touch. Let it sit over night an it starts immediately.

 

Since it doesn't fire, it should be simple to see if you aren't getting spark, or not getting injector clicking. Hold a long screwdriver on an injector with your ear on the handle end and have someone crank the engine. You should hear clicking as it cranks. Or get a noid light, and plug it into an injector connector and see if it flashes while cranking.
Does the tachometer needle move while cranking?

 

Hi Michael

Maybe the Wax in the Thermal Fuel Enrichment Valve on the Fuel Rail has Melted

Although it's Screwed into the Fuel Rail, it just goes into a Hole that is Blanked off so no point in taking it out as there's nothing to see

But the one thing that it does do is to Control the Vacuum if the Fuel Rail gets overheated

Less Vac = More Fuel

More Vac = Less Fuel

So if the Wax inside this has started to melt as it did on my Car, then it could cut off the Vacuum and lead to Over Enrichment until your Car has cooled down

This Fuel Enrichment Valve is really more trouble than it's worth and most Cars will run perfectly fine without it

So pull the Vacuum lines off it and either Join them together of simply replace them with a Single Tube and see if this solves the problem, as it did on my Car

Have a look at how I fixed it half way down page 87 of my 'Cherry Blossom' restoration Thread ('Sweet Smell Followed By Loud Bang') as She was getting too much Fuel

Sweet Smell Followed By Loud Bang! Fuel Enrichment Switch Had Melted Causing Over Fueling Making for hard starting

 

The Coolant temperature sensor sometimes get confused when the engine is in the process of cooling down. Try bridging it to take it out of the equation.

Paul.

 

One of mine did this, usually on 40+c days, when we stopped at the pub for a coldie.

I, for some strange reason, had the Ignition ON, pressed the Gas pedal to the floor once, and released, hit the starter, vroom.

Got into the habit, never solved the problem as such.

Another thing is the Ignition switch electrical section getting old, and dropping connection when in the start position. Starter works, but the Ign side has lost contact, gettimg very common now, BUT, not usually heat related.
I always fit a relay in the White wire to the +ve of the coil, thus eliminating that issue somewhat, and supplying a good 12v to the coil.

 

Thanks for all the leads- makes me ponder a lot...
- i'll do more checking of spark and injector firing during problematic starting. Not sure if tach is reading at that time. I'll check that when weather clears.
- the temperature switches - both coolant and air intake have be checked and are responding and i tried bypassing each of them to simulate a cold engine. No improvement.
- the vacuum switch on rail has been by-passed. I understood it was to to block vacuum to B regulator raising fuel pressure when hot - to counter vapor lock. The temporary switch on the pump let me flush the system when hot to make sure vapor lock wasn't a problem. I was working on an electrical replacement but have since moved on. (electrical temp switch in place of the vacuum switch, tied to a small vacuum valve that would do the same thing when the rail was hot).
- i have tried going to full throttle before cranking like setting a choke, but no improvement.
- i'll look into the ignition switch. Most confusing thing is the problem is so consistent, very predictable.

thanks to alll

 

While cranking, check for 12v at the positive post (white wire) at the coil. It goes thru the ignition switch to the coil(s) and other things, but goes thru several additional connectors on the way to the coil. Multiple chances for a poor/ corroded connection at a connector even if they look good. Could measure 12v static, but might drop dramatically under load. I've seen connectors work fine cold, but gain sky high resistance as current flows and they heat up.

Try also adding an additional ground directly between the engine and chassis. The stock setup has one strap from the engine to the motor mount area, and another between there and the chassis. Multiple opportunities for poor grounding.