308 straight gas V8 running rough when hot - no power

[Baja Burley]

We have a 308 in the lux and when it gets hot - 95*+ it starts missing and coughing under load, idles fine, just won't rev at all. We are working on running it cooler bu this only avoids the problem, not fixes it. Is it the gas somehow reacting to the high temps? Ign Leads heating up? I have limited knowledge in the gas area so helps out!!
Cheers
Luke

[want33s]

Has it got an electronic dissy?
If so, swap it and the coil for another one to try.

[date]

Sounds as if it is running lean. Not enough gas - makes it run hot, poor power as well. Check the filter on the gas converter. Also, look for kinked hoses.

[Baja Burley]

Mate its just had the gas system redone and certified last week, May be running a little lean.. I'll look into it. How do you adjust mixture with an IMPCO setup?

[date]

Not sure with the impco system. Firstly, I would check that you have power to the solenoid valves on the tank and the converter. Then, make sure that they actually do go 'clunk' when the system is powered up - this requires 2 people - 1 to try to start it and the other to check each valve. You may have some obstruction in the system. Then, take it to a gas man and get him to check the mixtures with his exhaust gas analyser.

[Baja Burley]

We definitely have both solenoids working A1. Otherwise the car wouldn't start. And you can hear them both when you flick ign on. In my experience (petrol only) it sounds like the engine is starving hardcore. Sounds Similar to a blocked jet in a carby. I guess I'll have to take it Into someone. Was hoping to have a crack at it myself but it sounds like I've got buckleys!

Could I bee looking in the wrong spot here? Maybe an electrickery issue?

[date]

Check the gas hose from the converter to the manifold. You may have a leak and under full throttle this would allow lots more air into the system. Check for leaks anywhere in the system, although if you have a small hose off it would affect the low end of the rev range, not so much the higher end.

[Baja Burley]

I'm hesitant to think its an air leak anywhere as it runs fine all the way through the rev range and only starts rooting up when it gets near 100* it has been leak checked by the gas man with his magic wand about a week ago to get the cert done too.

I hate fault finding...

[date]

When I said leaks, I meant air leaks, not gas.

Is it possible that you have a blown head gasket? I had a warped head on my old Range ROver years ago, and only when it got about 2/3 of the way to the red mark on the temp gauge did it suddenly lose power. When the temp resumed normal (about midway), full power resumed. Being a V8, I didn't know which head it was going to be, and of course, I couldn't check the compressions because it had cooled down and comp readings showed no problems. Eventually I took the most difficult to access head off and saw where it was blowing between the adjacent cylinders. Got it cleaned up and all was good from then on.

[Baja Burley]

I haven't checked for air leaks yet... Will get onto that. The heads seem to be seated nicely on a brand new gasket. Have done usual checks and all say gasket is fine.

Could the gas be overheating? The impco preheater gets farkin hot after a good run. We are attempting to put a ball valve in the water hose to the Pre heater to regulate the temp a bit. Just trying one thing at a time to see what works.....
Cheers for your thoughts Date, much appreciated.
Luke

[date]

The Impco pre-heater must get hot in order to operate. As the LPG is vapourised, it causes a massive drop in temperature and the converter will freeze into a solid ice block if there is no water to heat it up. Inserting a restriction into the water system will not help with your problem (which is not related to the gas overheating - it is the engine overheating). Does the engine overheat on petrol as well? If so, you have problems in your cooling system. The thermostat is the easiest to check - temporarily remove it and see if the problem goes away.

[Baja Burley]

It's running straight gas, so no petrol option. We have just fitted up a monster thermo fan which has helped a lot with temps. Its running at MAX 95* after a hard flogging in 1st for ages. Thats with an 82* thermostat. But still buggers up after a while of running. Would it be better to throw in a 74* Tstat?

I'm finding it hard to believe its a symptom of overheating alone, I've had engines a lot hitter than this and although they lose power they don't cough and splutter like this is. My little brain is about to implode!!

Luke

[Baja Burley]

Has it got an electronic dissy?
If so, swap it and the coil for another one to try.

It's a points dizzy if that helps.

Luke

[want33s]

Has it got an electronic dissy?
If so, swap it and the coil for another one to try.

It's a points dizzy if that helps.

Luke

Ahh..OK... I've seen qwuite a few cheap electronic dissys that crap out when they get hot... ProComp and Mallory come to mind...

I'd still be going over the points dissy and checking the point gap/spring, condensor and coil.

[bede fab]

have you checked coolant system ,thermostat etc shoulds to me as when it gets hot may be playing up with gas as it should run about 82 degrees thats my opinion only

[Baja Burley]

I know for a fact the coolIng system isn't quite up to the scratch for the big donk and I'm working on that...
I just want to know what is causing this perfectly smooth running engine to turn to sh1t as soon as it gets above 90*??

Next port of call is the electrics, plugs and leads. Everything else is brand spanking. New coil, new points, new condenser, new rotor button and dizzy cap. It's really starting to piss me off that I can't fix it!!

Any left field, out of the box suggestions? I've tried all the normal things..

[date]

Have you checked your ign leads? Get into a totally dark environment - a garage with the vehicle nosed inside, and start the engine. See if the sparks are lumping to earth or to each other. When you lose one cylinder from a V8, it isn't necessarily obvious - with a 6 or a 4, you can feel it rough, witn an 8, it often just loses power.

Have you closed your plug gaps down a bit? Champion recommend closing the gaps by 0.1 mm or more eg my vehicles are 1.0 - 1.1 mm recommended gap, but I run about 0.9 mm. This is also because they are such a sh1t to get to, I don't want the gaps opening up too much during their service life (100000 km). Champion also recommend one grade colder plugs .

Sadly, I cannot see how a higher temperature can cause gas mixtures to change. When it does go stupid, try isolating one plug at a time and see if the engine slows down. If, when you cut out, say No 4, engine speed doesn't change. Then, you have a place to look further. If it does this, then I would be thinking about blown head gasket which when the engine cools down, may reseal again until it gets hot again.

[rockcrawler31]

without knowing much about petrol engines and how their fueling works, my gut feeling would be a gasket. thermal expansion allowing cylinder pressure to bypass between cylinders or something then closing up as it cools.

[Baja Burley]

Gasket could be a possibility, we only just did the head gasket not even 20km ago.. But that means nothing. As much as I hate to admit it... Is the bubbles in the radiator the best way to check?

[rockcrawler31]

did you do both heads?

the bubbles in the rad are only one sign of one type of leak. Depends on where it's leaking to. Could leak from one cylinder to another, from cylinder to water, from cylinder to crankcase.

while the head is off get them crack tested as well.

Or just drive it and keep the temps down.

[Baja Burley]

Yeah we did both heads, haha. The heads are an absolute koont to get off as it sits up under the firewall so tight...
Gotta do what ya gotta do but... God I hate troublesome engines!!

I'd rather explore other avenues than have it off the road for a month; the old "while I'm here" syndrome will no doubt make it take it that long. Grrrr!!

[geoffro46]

points ignition usually arnt good enough for gas i would find an eleccy one of a blue motor and use that.