Non start spark present

XJ40
1

A non start for some time now.
I relate what was preceding.

This car has an lpg system. Prior to the non start the car would intermittently lack power and badly misfire on lpg. Especially when demanding more power (ex going from downhill to uphill) Switching to petrol solved to problem.
Later the car would have trouble firing up (standard on petrol) in the morning (dampness?) no trouble when hot.

Now nothing anymore.

Changed all the ignition components (sparkplugs, leads, rotor and distributor) Swapped the ignition amplifier thinking it was ignition related.

Tested for a spark and has spark

Tachometer moves up when cranking. So probably no cps problem.

So probably fuel related
sometimes it gives a slight puff noise (like being flooded) I even had it make a backfire.
Normally it can be emergency started directly on lpg, but also there nothing.
Could this be injector related? How can I test this?

2

When you attempt to start the engine on petrol do you hear the fuel pump give a prime to the fuel rail?

3

I used to keep a spare dist rotor in my toolbox at work at the dealer and one in the boot for my personal 1992 XJ40.

AJ6 engines are hard on dist rotors for some reason.

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4

Yes pump is priming. I had a problem with power to the pump and now the power to the relay comes directly from the battery

5

I have now 4 of them.
Could it still be spark related you think?
Does there exist something like a weak spark?

6

Fuel pressure at the rail?

7

Have to check but I don’t have a device to measure it, is there a way around it, or should I try to take out the injectors to see if they squirt fuel?
Many thanks for all the suggestions.

8

There should be a Schrader valve or similar on the fuel rail. You need to attach a fuel pressure meter to it and watch the pressure while cranking. It should be in the range of 40-50 psi. If it doesn’t reach that it won’t be enough to get through the injectors. Local parts store can probably loan one if you don’t have one. Doesn’t take long to test. Make sure you have a rag close by as typical there can be a bit of spray upon connecting or disconnecting. As on the XJ category to find where the tap is located.

9

There’s no schraeder on an AJ6 fuel rail. Rail has to be modified to fit one if deemed necessary.

10

I’ve been thinking this weekend. It seems to me the problem started with the lpg system and later went on to the petrol system.
Normally this car can be started on lpg directly. But is doesn’t do that.
Since spark is present I suspect the injectors to be the culprit
This is imho why:
The lpg system is a master slave system. The petrol injector says to the lpg injector when to open and close. the the start and duration is then modified by the lpg computer for the lpg injector to fill the right amount of gas.

But if one or more injectors don’t receive a signal. The lpg injectors related to the same cilinder won’t do a thing too.
I think Since the lpg system is a prolongation from the petrol system if there is a failing connection the lpg will fail first. And later on the petrol one too

But this is all theory.

I m still wondering how to test the injectors (petrol) if they all spray.

11

Just busted my own theory,

Took out the fuel rail with its injectors connected and put a little cap (from a bottle) beneath each connector. Took out the sparkplugs to make cracking easier because I’ve been cranking a lot lately

Result: all caps contained fuel.

So have spark and fuel.
Maybe need to check fuel pressure after all.

One more thought.
I put in spark plugs with a gap of 0.8 mm instead of 0.9mm and a ranking cooler (suggestion from a lot of Lpg forums. To make sparks plugs running lpg last longer and not to heat up so fast.) could this also play a role?

12

Little error above : Cap is beneath injector not connector

13

Check your firing order perhaps?

14

1:5:3:6:2:4
One being close to the radiator
Good you ask because I swapped 5 and 6 previously but still… a non start😔

Question: does it matter where on the distributor 1 is sitting or does only the order matter?

I noticed on the new spark plugs (put in after the non start) 5 was back whereas the others were all shiny. I suppose it is the only only one that’s managing to fire something.

Thanks for all the suggestions

15

It absolutely matters where the spark plug leads are. In order for the spark to get to the correct cylinder at the correct time of the piston travel. If you removed all of the leads at the same time you will definitely need to sort out the starting point on the distributor cap and the subsequent placement thereafter.

16

Maybe this is what’s happened with me swapping and re swapping. And why it even backfired on me one time.
Can anybody describe how they should sit on the distributor? Or maybe just describe where cil1 should be connected?
Many thanks.

17

A quick google search;

IMG_1172
IMG_11722160×1620 469 KB

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18

I’m so ashamed that I almost didn’t dare to reply. It was indeed the orientation of the distributor. It was 180 degrees off. :persevere:
The more so because I teach car mechanics at university.:face_with_peeking_eye:

So there you have it: there is a big difference between talking about theory and doing it in practice.

A lot of respect to the one pointing to the firing order and by extension to everyone on the forum.

2 Likes
19

Frederic, just look at it as a learning opportunity, not only for you but one that can shared for the benefit of others. Are you familiar with Rockauto.com’s “Repair Mistakes & Blunders”? These kinds of things happen to all of us. The only folks that don’t make mistakes are the ones that don’t do anything! Thanks for sharing.

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20

Hey that was me LOL