Hi all, hope you can help.
My 92 Daimler 4.0 has a misfire when the engine is warm and revved over 3000rpm. All runs fine when she is cool, but as soon as she's at operating temp the problem manifests.
I've tried, new HT leads, new injectors and seals, tested vacuum hoses and air leaks around the MAF and throttle body, new rocker cover gasket and spark plug seals, removed intake manifold, renewed all gaskets and re fitted, torqued up all fine, and a new sensor that sits on the thermostat as I'd read that could cause the issue, but no joy!
Any ideas??

Thanks in advance!

Steve.

 

Hi Steve,

It's really difficult to diagnose from afar, but I have two suggestions:

1. measure your fuel pump pressure at the rail (cold and warm) and
2. measure your engine's compression for each cylinder with all sparking plugs removed.

from



1.11 Fault Diagnosis (page 22)

a) Engine will not start

1. Defective coil
2. Faulty ignition amplifier or fuel ECU
3. Faulty injector(s)
4. Loose or broken electrical connectors or leads
5. Damp ignition leads, distributor cap
6. Battery flat, corroded terminals
7. Defective fuel pump, vapour lock in pipes
8. Blocked or split fuel filter
9. Valve timing incorrect
10. Valve clearances incorrect
11. Valve seats burnt, valves sticking
12. Broken piston rings
13. Plug leads incorrectly connected
14. Faulty, jammed starter motor
15. Sparking plugs defective or gaps incorrect

...

d) Engine misfires (your problem)

1. Check items 1,2,3,5,7,8,9,10,11,12 & 15 in a) above
2. Weak or broken valve springs

 

Hi,
I have had problems with coils when the engine gets hot and they start to de energise, may be worth swapping for a donor one first to try.

 

Hi Steve,
forgot to add I also have had dodgy spark plugs that see ok when first started engine, but play up when they get hot too.

 

If the plugs are acting dodgy also, you might consider a new set of plug wires.

 

Thanks for all the advice guys.
Tried new HT leads and plugs today, same fault.
Interestingly she fires up and revs right up to the top of the range when cool, but misfires when hot, and the hotter she gets the sooner in the rev range the misfire is seen, starting at 4k when warmish and ending up being seen at 1500 - 2k when hot, and won't rev above this range, just coughs and backfires.

Is it starting to look horribly like a head off job?????

Thanks again

 

I'd exhaust all of the steps I posted above before I would pull the head.

Is it one cylinder or all cylinders misfiring?

Are you absolutely +tive the fuel pump or regulator isn't at fault? What you last posted reads like a fuel starved engine. Bad pumps are known to get weak when warm.

Only step 11 (I posted earlier) would require the head to come off and a compression check after the engine is warm is in order to confirm step 12.

I would run the engine with the valve cover off and a sheet metal dam to keep the oil where it belongs in order to inspect valve springs and cam lobe operation.

Just my 2₵.

 

Hi guys, thanks for all the advice so far!
I've discovered a possible issue whilest changing the lambda sensor, one of the cats looks to have collapsed inside the down pipe at the front. Now I'm GUESSING this cause a misfire as the engine gets warmer as it will prevent the exhaust gasses escaping??
I hear the fuel pump is a bit of a bugger to replace, anyone know how to test it first?

Cheers again!!!

 

H,

I don't know if the collapsed cat is causing your problem, but it's definitely affecting your engine's ability to breathe - which is probably affecting your engine management system.

1. You probably have other issues, too. Did you investigate your air flow meter (MAF)? I read where the connector alone can be troublesome.

I may have read that at Jag-lovers. I think you will have to join (free) in order to read it. It is very good.

- Jag-Lovers' xj40 Ebook: Index Page -



2. Here is what Hayne's has to say about testing the fuel pump.