Chevrolet Repair: Chevy diesel 6.2 stumbling, chevy diesel, chevy blazer

Question
Hi Van,
Thanks for any help you can give me.
I have a 1989 Chevy Blazer full-size with a 6.2 diesel engine, 4spd auto trans, and its 4x4.
It has a lot of miles - around 200,000 miles. Never had an engine rebuild, although the tranny, driveshaft and rear end are fresh rebuilds about 6 months ago.

It's always run well and maintained regularly. It's had an easy life, not taken off-road and not towed more than a motorcycle once in a while. Mostly highway miles.

A couple of weeks ago, I noticed that sometimes it would idle rough. Like it was stumbling a bit.
Last week, I noticed that when I was driving around slowly, it would sometimes surge, as though I had stepped on the gas pedal (but I hadn't)...and it wasn't the transmission shifting down. I didn't think much of it.

Yesterday night, I notice that as I'm driving around, the whole truck started to vibrate quite noticably and lost a bit of power.

This is how I can best describe it. Driving around slowly, under 15MPH was fine. When it got past that speed, it would start to vibrate. It would vibrate more when I'm pushing the engine to accelerate. Just before it would shift up to the next gear it was vibrating like crazy. It felt like my tires were square, although I got and confirmed they were fine!

Once it was crusing at around 30MPH in the city it was not too bad.
When I was coasting down hill, in gear, it was fine. In fact it went up to 50MPH downhill and very little vibration. When I popped it into neutral downhill, there was no vibration at all...smooth coasting. So I know the tires were fine, rear end, etc?

So it's not a drivetrain thing, it's only when the engine is engaged and it's pushing. When the engine is not working hard, the vibration is almost gone.

I'm thinking it's some of the cylinders not firing? How would I go about testing this?
Being a diesel engine, I can't just pull a plug and check for spark.
is there a way to test the injectors or the injector pump?

Any other theories as to what it might be?

Thanks so much for your time.
Regards,
Alex.

Answer
Change the fuel filter first.
Then there is a way to isolate which cylinder, and that is cracking the retaining nut of each injector, one at a time. That is messy, and can be kinda hard, but when you find one that doesn't make as much difference as the others, that is the cylinder. Doesn't mean the injector is bad though.
Removing and swaping a couple injectors can tell if it is an injector, because the miss will travel with the injector.
if it stays with the same cylinder, then there could be a valve problem, or an injector pump problem.
A compression test can help decide if it is a valve problem, but it takes a different compression tester than a gas engine.
Change that filter first.

Van