GM 6.2L inconsistent "ticking" noise?

[regraham]

I have a baffling problem with my 2007 GM Denali pickup c/w 6.2L engine. It has an ECO propane vapour fuel injection system installed. I have 118,000 km on the vehicle and it has operated on propane (duel fuel system) since new. I use Amsoil 5W-30 in the engine.

For the past few months the engine occasionally makes a "ticking" noise (tic,tic,tic…….) when operating on propane and noticeably at idle, when I am standing outside the vehicle. It usually only occurs when the engine is well warmed up, for e.g., if I have driven on the highway for an hour or more. It is not a consistent problem, only occurring some times, but lately more often. The strange thing is that if I switch the fuel system to gasoline, the noise almost always goes away within a minute or so. The engine still pulls strong and does not mis-fire. For the past few days I have driven the vehicle under similar conditions and lengths of time but on gasoline exclusively and I have not been able to duplicate the problem. I have had a couple of different skilled mechanics listen to the engine when the problem is manifesting itself. I have had suggestions such as rocker arm noise, lifters, exhaust leak, etc. but the fact that it is inconsistent and not on gasoline baffles me and them. When I first start the engine in the morning (it always starts on gasoline) there is a few seconds of "rattling" noise which I assume is the lifters, until the oil has fully circulated?

Any suggestions?

[C3H8]

Hi Reg,
both my 2000 and 2007 6 litres had a similar problem right from the day I got them on. They were pre-owned at the time with about 60 to 80K on the powertrains. Cold starts had an audible ticking or rattle for the first 30 to 60 seconds of operation. It was louder in the winter and less noticeable in the summer. This was on gasoline initially but also occurred on propane. I did quite a bit of reading on it and found many sources that pointed at the pistons being the cause. I talked to several GM techs that confirmed these engines had a tiny amount of piston slap when cold right from brand new, but in GM's estimation was not considered detrimental to the life of the engine.

In your case you have a 6.2. It sounds like this was solved on the 6.2 as you have not noted it prior to this time. I suspect that the reason for the tic on propane is the dry fuel vs liquid. Gasoline is going to coat the walls and rings a little muting or preventing the tic. Dry fuel does not. The liquid may also provide a small amount of cooling during the intake cycle to stop the tic. I am going to venture a guess that the tic will increase over the next 20 or 30K to the point where it becomes more audible and consistent on both propane and eventually gasoline. I am not 100% sure but I believe some owners of the 5.3 and 6 litre engines in the US started a class action law suit against GM for this cold start problem. Not sure what happened or even if it was settled yet.

[regraham]

Thanks for your suggestions Randy. You are probably right as to why it "tics" on propane but not on gasoline. Last week I made a 75 km trip to Ft St John driving on propane. When I got there it was ticking. After a couple of hours I drove home and again it was ticking when I got home. At the suggestion of the local GM shop I have been driving it exclusively on gasoline for the past few days. Tonight I made the same trip to Ft St John, on gasoline. It ran fine, pulled the hills strongly and no ticking.

Now what? What is your experience with Top End Oilers? I am wondering if I should go that route?

[C3H8]

My personal experience is very low with it Ron. I know that many of the dealers have been installing them on the injected systems with what they claim is good success. The only problem is that the actual km's on the units is still limited so we can't claim with 100% certainty that they work. There true purpose is to provide lubrication to the intake valve surfaces to prevent recession, however I would also expect that would provide some lubrication to the cylinder walls also or at least cushion the walls a little. The system is called Flash lube and I believe it actually originated in Australia where propane continues to be vey popular. The system is popular in the UK also and heavily promoted by the installers. The only downside is purchasing the lubrication product. I believe it uses about 1 litre every 10,000 km's. Maybe some of our members have direct experience with it on high km engines and will offer their view.

[storm]

I have a flash lube kit on my V8 Commodore. I fitted it when Australia removed Leaded petrol from sale and only had Unleaded available. It certainly does help with valves (and associated parts) and it does stop ticking tappets but I am not sure if it helps with piston slap and wear of pistons and cylinder walls. When fitting it you are supposed to set it to allow a certain number of drops every 5-10 seconds and that is specifically for valves. I suppose if you increased it slightly it may help pistons/cylinder walls.

[regraham]

Thank you C3H8 and storm for your suggestions and info on Flashlube. I ordered in a kit and installed it on Jan 30th. The next day I did a 100 km trip operating on propane and guess what. No ticking noise when I completed the trip and had it idling before I shut it off. The kit was easy to install and set up. I set it at the prescribed 8 drops into the sight glass per minute and it seemed to use about 1 ml per liter of fuel as recommended. Now I wish I had installed the kit years ago.