Engine problem and I'm out of ideas

[351ciofgrunt]

Hey guys,
I know its not a ford but i'm having a problem with my GQ patrol TB42 carbie. and I'm out of ideas.

Symptoms
- car idles okay with a slight miss
- soon as you start driving it there's shuddering, surging and also some backfiring through carb, kind of feels like when a fuel pump is gone and the cars starving for fuel. Problem just started one day.

What I've done so far:

Fuel
First replaced fuel pump, then replaced carbie which made no difference, then eliminated fuel system entirely by ripping off fuel system and replacing with straight LPG gas (impco model E and 425)
- made no difference to problem

Compression
-compression tested all cylinders and they are within 5 psi of 165psi

Spark
Replaced points, ignition coil, checked and gapped spark plugs, checked ignition timing

Engine oil looks good and clean
Coolant looks good and clean

I'm out of ideas

I suspect a cracked cylinder head, perhaps a crack between two combustion chambers however it didnt show up on the compression test, but maybe its small enough to only cause a problem under combustion pressures and not cranking pressure?

[Windsor220]

Air leak into the intake?

[351ciofgrunt]

Originally Posted by Windsor220

Air leak into the intake?

yeah its a possibility, i'll have a look tomorrow

[XAJKX]

Originally Posted by Windsor220

Air leak into the intake?

Thats my guess too.

[cs123]

Might also want to check mechanical spark advance.

[Jeddak]

Its a long shot ,however, leaking brake booster? it gets its vacuum from the inlet manifold and can lean off the fuel mix in the manifold .usually problems here can be heard when you put your foot on the brake slowly and you hear a sucking sound down near your feet (engine running of course!)

also a trick to try ...get a pair of needle nose pliers or better still compression circlip pliers and squeeze shut in turn each vacuum hose. any difference in idleing will show you the problem

another .... the advance unit on the distributor? does it work ?and also hold its position?

last but defiantly not least ...plug leads .. have them tested or replace them as a set. the leads can burn away inside the lead outer and the problem will not be visible
:

[mik]

coil lead? condenser?

[smally289]

Check the rotor in the dissy. These things have a spring loaded attatchment that act as a rev limiter, its possible one or both of the springs have fallen off or stretched.
Worth a look.

[spannerman]

have a look at the spark plug leads

crack in distributer cap or carbon traces in dizzy cap that can cause cross firing

[PALE ALE]

Did you change youre condenser?,have you checked you're coil lead?(resistance) I own one of these too, I had ignition dramas like you are describing, but only on LPG. took me months to solve it. First, you have to use genuine Nissan points, they are vented(have a hole through the contact, to cool it) the generic/aftermarket ones dont have this hole, get hot and will burn out after about 5,000 km's. Second, the gap between the rotor button and pins in the dizzy cap is too big(about 6mm), you need to reduce this. I bought an aftermarket rotor button from outlaw speed shop here in Adelaide that has been modified (lengthened), the gap is now about 2mm. These two things solved all my ignition dramas. But as I said earlier, I only had this problem on LPG, on petrol it ran fine

[Wally]

If it was me I'd be putting on a cheap vacuum guage and seeing what the needle is doing. Then I'd do a running compression test to see if the exhaust, valve lash, or inlets are the culprit.

You may well have slipped a tooth/cog on your valve timing.

Backfiring through the carby is usually spark timing

[Riksta]

www.patrol4x4.com.au is full of Patrol enthusiasts who may be able to help if the people here can't.

[whales]

What about a blocked fuel filter. Same thing happen to me with a Mitsubishi and it was a block filter.

[yift]

sounds like a possible blocked exhaust, if its an unleaded modle then check the cat.

[351ciofgrunt]

Hey guys Problem fixed!!

Thanks for the good ideas.

It turned out to be a combination of:

* dodgy spark plug lead (timing light showed it was only firing 70% of the time at idle and not at all when revving)

* vacuum leaks (old hoses gone rock hard and cracked)

The lesson i've learned is dont overlook the basics!!

I'm so happy... :-)