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Need to know WHY I have fixed a Mates engine


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This is odd to my mind.

Mate has a 3.5 flapper efi engine, in a 90 -he bought it with a misfire, over the last few months I have suggested stuff over the phone

and been down to have a bash at it, several times and its been up to me - several times :(

Tonight he phoned to confirm its now seems fixed, ..........and although I know now what the problem was I can't work out WHY :)

I will explain, ...........then I expect someone to give me a sound answer why :lol:

Basically it ran fine when cold, and then OK as it warmed up

Once fully warmed up.... about 20 - 60 mins after this (lets say after the thermostat opening) it developed a big misfire.

Let it cool down all OK again and circle contuines...

Prob not everything but as a guide....................

NEW Plugs NGK, Leads Cap Rotor arm (genuine) New AMP COIL (Genuine) Fuel Pressure ALWAYS OK (bear this in mind) FULL Diagnostic check - everything fine, cahnged for known goods ECU AFM TPS Steering relays Main relays fuel pump. Borrowed and fittted my New harness - nope refitted old gave me back, Cleaned tested injectors, Replaced Resistor pack known good, Checked timing, checked Plenum flap, had dizzy apart and rebuilt reset, changed dizzy also for known good, Blocked 9th Injector, Changed fuel pump known good spare, Fitted Cheap filter return pipe, changed Coil to Dizzy lead, Checked Airleaks, full compression test, head gasket tests,Checked Ign Advance discconected replaced known good Extra Air Valve etc etc etc

and a shed load more .............and exactly the same :(

Last weekend he arrived around the house again, "Warm Mit Misfire" :(

I just stared at it :huh:

I then noticed the Fuel Filter (which was a genuine 3.9 one) was fitted wrong way for fuel flow,

I disconnected it and it was dirty as anything on the IN Side ie fuel TO the engine, and as said the filter was back to front

I had a spare so fitted it, and shoved 2x tins of Injector flush in the tank

By now it was cold, it started up lovelerley and off he went both of us wondering :huh:

And now (Wends) he has done 400+ miles ...............all's well.

So, It seems to have been the filter - but

WHY did it run till 20-60 mins past warmed up perfectly fine ?

Even with Misfire Fuel pressure was perfect and no change - ie not blocking fuel flow :huh:

Why then ok when cold again ?

What effect does running a genuine 3.9 HP Filter in reverse maybe do ?

WHY did the little filter in the return stay clean - if dirt not present then why the misfire ?

Delighted to have sorted it for him, but puzzled as to maybe why ?

Odd :lol:

Thoughts then ?

Nige

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HfH

Was going to make a WAG, but then re read the post.

I'm stumped.

Would have expected more fuel consumption when cold, so blocked filter having more effect then rather than the other way round.

Only thing I can think of is the dirt fell off the filter when the pump was stopped, allowing fuel to flow through the filter. MAybe there is cr*p in the bottom of the tank being dragged in to the filter when running?

The back to front filter still filters the fuel, hence the return staying clean.

When the engine was warm and the fuel pressure checked, was it at idle speed. Was it misfiring at idle, or only on load?

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How about this for a shot in the dark:

Working on the basis that it's got to be to do with heat soak and the slow rise in temperature of the filter - combined with the back-to-frontedness and the nearly-blockedness then:

1. The filter is getting a little bit soft when hot (assumption)

2. The filter is dirty and will be suffering a pressure difference across it's filtering layer. (fact)

3. If the filter is the correct way round this pressure difference, combined with the softening will cause the filter to become smaller in diameter.

4. If the filter is the wrong way round this pressure difference, combined with the softening will cause the filter to become larger in diameter - thus closing the gap between it and the outer container and reducing the fuel flow....

Turning off the engine will allow the filter to spring back to normal shape and then allowing it to cool will allow it to set like that, hence it works until it once again gets hot enough to go soft.

Roger

(tin hat on :D )

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[

Yep, It werid isn't it ?

The misfire when it came was from tickover right up the range - you couldn't drive through it

Fuel pressure checked right through rev range - no probs

When we changed the pumps I shone a torch and the tank bottom was clean as a whistle

The crud was ONLY at the filter joint and as I said filter back to front

Stumped :lol:

Nige

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Could've been the injector cleaner.

Or, could've been something daft like the fuel filter creating an airlock (they must have a correct direction of flow for a reason) that gets bigger as it runs for longer, then bleeds down when you turn it off and leave it.

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OK Last throw of the dice.

Fuel flow insufficient to allow for cooling effect of the return to the tank.

Once engine warm, the reduced flow through the system allowed the fuel to get too hot???

OK I give up. No Idea. Haven't a clue. Completely stumped. In the dark.

It's bad enough trying to diagnose these systems when they are still broken let alone when they are fixed. Still look on the bright side, something else is sure to cr*p out again soon!

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Stab in the dark, but if the fuel filter has a build up of sediment/carp in it by the time the engine was warm it would have been well disturbed and started to block the filter enough to reduce the flow. when the car cooled down the sediment/carp settled again so it ran ok.

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Nige, I know you said the fuel pressure was checked through the rev range but I assume that was with the vehicle stood still. Did you check it while driving?

It may well be that although the fuel pressure was fine the flow was just a tad on the low side, did you ever see any change in the return quantity?

Could be that a tad low flow would eventually cause pressure to drop, fuel temp might also have affected the filter carp and reduced flow.

Puzzling.

jw

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Interesting Posts :rolleyes:

This has been doing my head in...so

I have spoken to a very very very clever bloke who 'knows a thing or 5' of V8s.

He sort managed to explain what was the issue, and why it did what it did.....and frankly it sort of melted my brain.

I then phoned and discussed it again with Ian (BBC) .........and between his mine and the other explaination I have sort of got it now :lol:

It was the filter and it is fixed :)

I will let all stew for a day or so, and then I will try to explain the wisdom imparted into me :P as to WHY :lol:

Nige

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  • 2 weeks later...

This may not be the answer to your quest, but might be?

Had a similar problem with a "Flapper" system that I had installed on my 110 V8,

The engine would run fine for days, then all of a sudden when it got hot it would misfire as if it was out of fuel.

Open the bonnet & let it cool down & off it would go again sometimes for months.

Eventually I tracked it down to the wires from the distributer base plate, they would occasionally move too far under load

& the engine would play up.

Replacing the base plate & Hall effect sensor cured the fault. I sold it 12 months ago & according to the chap I sold it to it was

running around Russia up to a few weeks ago without any further problems!

Tony

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This may not be the answer to your quest, but might be?

Had a similar problem with a "Flapper" system that I had installed on my 110 V8,

The engine would run fine for days, then all of a sudden when it got hot it would misfire as if it was out of fuel.

Open the bonnet & let it cool down & off it would go again sometimes for months.

Eventually I tracked it down to the wires from the distributer base plate, they would occasionally move too far under load

& the engine would play up.

Replacing the base plate & Hall effect sensor cured the fault. I sold it 12 months ago & according to the chap I sold it to it was

running around Russia up to a few weeks ago without any further problems!

Tony

Nope no dizzy

Will post answer tommorrow

Nige

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