3.5 V8 Rough running > 80kms on LPG & replacement

[StuRR80]

Happy New Year all!

I recently had the Rangie tuned and the LPG working again. When doing the tune up, the specialist carb/gas mech said a cylinder on the left bank had low-ish compression and the spark plug was oiled. He said the engine would need a rebuild. Now, I don't have any money to play with for a rebuild but I do have a parts car with an engine which I was told was ok. What should I check to make sure I'm not replacing a slowly dying engine with an even more "dying" engine?!

Also, when running on LPG at speeds above 80kms, there's a miss or something which makes it run roughly, although not consistently. The mech said it was because the engine needed a rebuild (oil on spark plug, etc). I know LPG needs a "fatter" spark for run well on. It seems to run really well on petrol. I would assume the spark produced is always the same, no matter what speed?

I've replaced the battery (needed a new one anyway), the leads (although they're only Bosch, the mech said they were fine) and the coil (to a Bosch GT40).

I would have assumed the roughness would occur all the time, at all speeds but it's certainly only noticeable above 80 kph.

Any ideas?

Many thanks

Stu

[PhilipA]

Basically you should check the compression on any engine before fitting it.

Unfortunately there is also the issue of worn camshafts on rover v8s.

I take it the parts car does not run, seeing someone has told you that it is OK. So if i were you I would get it to run before going to the trouble of swapping engines. The whole car does not have to go, only the engine.
Regards Philip A

[bee utey]

Chemically and physically LPG is not petrol. The operational difference as far as you are concerned is the voltage required to ignite the mixture. It is higher on gas in a dual fuel engine unless the plug gap is reduced. Spark "fatness" is not a good criteria. Gap and voltage is.

The best answer for a points ignition is to reduce the spark plug gap to 0.6mm or 0.7mm. If it is an electronic ignition (fuel injection type) then the Bosch GT40 coil is incorrect and will not function correctly. Bosch BIC290 is recommended as a replacement. Your gap then should be 0.7 to 0.8mm

[101RRS]

I would run your engine on gas for a while and see if your plug is still dirty. Did your mech actually do a wet and dry compression test or just look at the plug.

I rebuilt a subaru 1800 engine and did something wrong and one of the clylinders ended up with lowish compression - never caused an issue and ran for another 250,000km before I sold the car. My point is I really would not worry about it at the moment - if it plays up later yes you will need to replace the engine but if it doesn't you have lost nothing.

Garry

[StuRR80]

Chemically and physically LPG is not petrol. The operational difference as far as you are concerned is the voltage required to ignite the mixture. It is higher on gas in a dual fuel engine unless the plug gap is reduced. Spark "fatness" is not a good criteria. Gap and voltage is.

The best answer for a points ignition is to reduce the spark plug gap to 0.6mm or 0.7mm. If it is an electronic ignition (fuel injection type) then the Bosch GT40 coil is incorrect and will not function correctly. Bosch BIC290 is recommended as a replacement. Your gap then should be 0.7 to 0.8mm

Sorry, I should have mentioned it's a twin-carb engine with an after-market Crane Fireball ignition.

I'll check the gap and reduce if required.

[StuRR80]

Basically you should check the compression on any engine before fitting it.

Unfortunately there is also the issue of worn camshafts on rover v8s.

I take it the parts car does not run, seeing someone has told you that it is OK. So if i were you I would get it to run before going to the trouble of swapping engines. The whole car does not have to go, only the engine.
Regards Philip A

The car does run but I've only managed to drive it onto and off the trailer. It doesn't currently have wheels, so I can't take it for a run around the farm to check how well it runs.

He mentioned the oil could be from the rings but he also said it could be coming through the valves from the heads? Is there a way to "easily" check which way the oil is moving?

[StuRR80]

I would run your engine on gas for a while and see if your plug is still dirty. Did your mech actually do a wet and dry compression test or just look at the plug.

I rebuilt a subaru 1800 engine and did something wrong and one of the clylinders ended up with lowish compression - never caused an issue and ran for another 250,000km before I sold the car. My point is I really would not worry about it at the moment - if it plays up later yes you will need to replace the engine but if it doesn't you have lost nothing.

Garry

He didn't say which tests he did, unfortunately. He did say it might last another 5 years but then it might only last another ????......!

[LoveMyV8County]

I wouldn't worry too much about low-ish compression unless it is very low.

What spark plugs are you using? I have had problems with ignition missing at certain speeds on other vehicles when the plugs are not the right ones. LPG burns hotter than petrol and the plugs need to be able to cope.

I use NGK Iridium ones on my LPG carby V8 with no problems.

Chris

[101RRS]

He did say it might last another 5 years but......!

Having low compression in one cylinder is probably not going to impact anything else and you probably won't notice the slight drop in performance - so as long as it still runs OK worry about it in 5 years.

[bee utey]

Having low compression in one cylinder is probably not going to impact anything else and you probably won't notice the slight drop in performance - so as long as it still runs OK worry about in 5 years.

Had low compression in one pot on my V8, when it got suddenly worse I popped out the piston to find a broken bottom ring and no top ring.. The engine builder had left out the top ring. Still ran OK just a bit uneven at idle.

Another tip with plugs, you should run plugs that are at least 1 heat range colder than stock, otherwise hot performance will suffer.