Cautionary Tale: High Mileage Engines with Aftermarket Turbos

[britannic]

FYI, I'd like to share my experience to help you avoid similar problems:

I previously posted that my timing chain had over 1" of play and that dual idler gears were fitted. After installing the gears, I had to set the pump timing about 1/16" to the passenger side of timing cover mark to get TDC @ 1400-2000rpm.

During the gear installation, I decided to winterize the truck in preparation for a high altitude camping trip, so the glow plugs were pulled. #3 and #6 glow plugs had melted tips which is evidence of pre-ignition, all of the rest were in good shape.

Since I have always run TDC @ 1400rpm with these glow plugs, my conclusion is that at certain engine speeds, the worn chain allowed enough play that an additional "?" degrees of advance was being added to the injection point on these 2 cylinders.

During driving when the chain was still installed, the engine would vibrate quite strongly in the 2300-2800rpm range and it wouldn't sound right. Since the gears were installed, the engine is smooth at all speeds.

YMMV, but if you don't know the condition of the timing chain on a high mileage turboed engine, use caution when advancing the timing; higher boost pressures ensure a very hot, quick and consistent burn and if the chain allowed certain cylinders to be too advanced, it's unlikely that this would show up on the EGT meter - it sure didn't on mine.

I just got back from that 600 mile camping trip involving WOT driving at altitudes of 8700', EGTs occasionally peaked at 1100F and boost levels over 14psi. I checked my glow plugs afterward and they are in perfect shape, thanks to the consistent timing provided by the dual idler gears.

[ 10-08-2003, 11:27 AM: Message edited by: britannic ]

[gmctd]

Were the glow plugs replaced previously, and if so what series were the replacements - AC-11g, AC-60g, or JK's extended glow-series?

jd

[britannic]

60G replacements all in excellent condition apart from #3 and #6.

[G. Gearloose]

Thanks for the heads-up!

[More Power]

We did a Phaser gear drive product review a few years ago. In it, author Scott Boelman noted that his 1994 6.5TD (equipped with a 5-speed manual) produced a vibration in the range indicated by Britannic. Scott was pretty sure the vibration was due to a transmission problem, and was preparing to remove and disassemble his trans to find the cause.

What Scott discovered was that the vibration disappeared after installing the gear drive timing set, even though Scott's 250,000 mile timing chain was still within GM's liberal wear specs.

MP

[britannic]

Originally posted by More Power:
We did a Phaser gear drive product review a few years ago. In it, author Scott Boelman noted that his 1994 6.5TD (equipped with a 5-speed manual) produced a vibration in the range indicated by Britannic. Scott was pretty sure the vibration was due to a transmission problem, and was preparing to remove and disassemble his trans to find the cause.

What Scott discovered was that the vibration disappeared after installing the gear drive timing set, even though Scott's 250,000 mile timing chain was still within GM's liberal wear specs.

MP There's no doubt in my mind that the timing chain design was a blind spot in GM's design. Ford and others saw fit to use gears, whilst GM carried on with gasoline tech. trying to drive the high load produced by the IP pump.

The rattle from my timing case screamed timing chain wear, but I just couldn't believe that such a short chain would ever wear that much and decided it was a loose baffle in the top of the timing case.

I became suspicious, when it proved harder and harder to time the engine; I would test, set and retest and get different results at different rpms each time - taking up to an hour or more. With the gears installed, the timing was accomplished in 5 minutes and is absolutely rock solid.

I believe that installing the higher pop pressure injectors with the 4911 spec pump, increased the pump load on the timing chain and accelerated the stretch to the level I found upon disassembly.