Low Mileage 6.5 Rough Idle in Closed loop.

silveradosebak50 · 2025-10-05T20:45:46-0400

Hello everyone,

long time listener first time caller. I recently acquired an absolutely MINT 1995 Silverado with a 6.5L Turbo diesel. only 46,000 Miles! You could eat off of it. An old man owned it and it lived in his garage and he never drove it. It is as close to new in every aspect as it can be.

The bad...i'm trying to solve a rough idle issue. here's what I know

no check engine codes
runs smooth down the road from what I've driven it
only happens in Closed loop when warmed up
if it warns up and starts idling rough and I unplug the coolant temp sensor and put it back to open loop it goes back to running as smooth as a sewing machine
the PMD was replaced and relocated to a cooler spot
that being said I still swapped out the PMD, resistor, and harness with a new one to be sure
new fuel filter and fuel

I'm trying to think what Closed loop sensors would cause this. MAP, MAF, IAC? It has to be some sensor that only comes into play when the engine warms up. someone with more experience with these 6.5s please give me some ideas.

any help is greatly appreciated.

57diesel · 2025-10-06T10:41:45-0400

Have you taken apart and cleaned the grounds? Especially the one on the right back corner of the intake? That's the point where most of the engine sensors ground so an issue there will cause all sorts of bad readings.

Have you installed a clear line on the return off the injection pump to look for bubbles?

Have you looked for an old scan tool that will read live data? Many of the good ones did. They don't tend to have the timing set functions but they will read data which would be really useful here. I see things like the OTC Genysis tools for sale all the time ranging from $50-200 depending on what's with it and who's selling it. Those tools tend to end around 2005 so many 'mechanics' have not used them in years. Even DIY people that bought them may be selling them because even they too moved on to newer junk.

silveradosebak50 · 2025-10-06T11:46:12-0400

I have not yet cleaned the grounds I will do that next. It would make sense if one more sensor was not getting good continuity to ground but only in play when in closed loop.

I did not install the clear hose to look for bubbles. The fact that it instantly clears up or appears by putting it in and out of closed loop tells me that it's not air in the lines which would present itself more across the board.

I did just get a scan tool. I have a lot of OBDII ones but didn't have an ODBI. It just came today so I will have to see what I can read from it next time I am at my shop.

Low Mileage 6.5 Rough Idle in Closed loop.

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