Violent Shaking, but strictly @ 1600-1800rpms

[Gypsy3000]

1st post: all respect to the grey and grizzly!
My first Cummins. Wrenched on many of my prior Ford 6.9/7.3 idi's, P-strokes, etc. Minor Soob expert.

Just bought a '97 2500 4x4, 47re, ext. cab. 350k miles. Price was low due to a very particular shaking problem.
Conditions:

• Only between 1600-1800 rpm's. Not above, not below (unfortunately, that is smack-center 55mph in O.D.),
• Occurs in all gears but only during low to medium acceleration,
• Does NOT occur at all during WOT, hard-accel, while climbing through gears,
• During WOT, truck seems to accel slowly through 1st & 2nd, but when it shifts into 3rd gear it suddenly has much more accel power.

Anyway, needed a tow vehicle, it seemed to drive okay outside of 1600-1800rpms, and the price was essentially half comparable market. Convinced it was trans-related, I bought it and immediately drove it "home" the 600-miles from SF to Baja. Above/below 55mph in OD ran very smooth; no issues & >18mpg. [What is typical City/Hwy mpg's?]

However, yesterday my very-trustworthy local trans shop said that he has owned/driven many 5.9's and he isn't convinced that this is a trans-related problem; he said that compared to others mine is noticeably under-powered and that the throttle response isn't normal. He enlisted a local Cummins specialist to ride-along for a diagnostic test-drive. He, also, was uncertain which system is causing this rpm/throttle-specific violent shaking. The fact that it instantly corrects itself above 1800rpms is confounding to all of us.
Consensus: given that I've already purchased a Goerends 3-disc TC/rebuild kit/shaft & Transgo Shift Kit ($$$), proceed with the trans rebuild and go from there. Regardless whether or not I prematurely misdiagnosed the shaking as a trans problem, given the discounted purchase price and 350k miles there's no harm investing in a rebuilt & upgraded trans.

In anticipation that the shaking problem is neither cured nor diminished by the trans rebuild, I'd appreciate any knowledgeable input:

• Fuel-system related? Injection pump? I've had to pull/flush entire fuel systems before [WVO-converted Fords, bad-gas sedans, etc]. However, to me this does NOT act fuel-starved = above 1800rpm's runs very smooth,
• At around 1600rpms it abruptly starts bucking HARD then around 1800rpms stops as quickly as it started. Given the distinct effect of throttle-velocity, combined with no GRADUAL vibration increase-decrease, I'm inclined to eliminate driveline suspects: wheel-balancing/rim/tire/front-end/brakes/diffs/bearings (I'll review symptoms of drive-shaft imbalance).
• Harmonic Balancer?

I know zero about Cummins control systems.
I need a reliable tow vehicle YESTERDAY.
Any qualified input greatly appreciated!!!

Caveat: I commited to this purchase while suffering from undiagnosed covid; very foggy-headed, usually fair judgement downgraded to poor... ("Fully" recovered now, but NOT fun).

 

[crash68]

Start looking at boost pressure when this shaking occurs, also fuel pressure. Could be something mechanical like motor mounts.

 

[Gypsy3000]

Thanks for the speedy reply!

I think the cummins tech mentioned a potential boost-pressure issue during test-drive. My Espanol is so-so, but "presione del turbo, e sistema de metra por controllo" etc.

Re motor mounts: Maybe, but given the abrupt and specific rpm window of the shaking? I'd think that the vibration would be more tapered. Also, at WOT wouldn't bad mounts = worse reaction, not better?
Anyway, it is possible so I'll have my trans shop inspect them.
Thanks!

 

[Gypsy3000]

Problem solved & solution confirmed:

- 70% Exhaust manifold gasket leak,
- 30% Trans internals or Torque Converter.
[Actually, given that I had the trans/TC work done BEFORE the man gasket, I can't be certain that the gasket wasn't the entire problem. More a 'gut-sense' of reduced shudder when driving post-op trans, pre-op gasket..]

Picking up truck in the morning.
Old Mexicano home-shop mechanic replaced the gasket, says it runs great now.
$150 parts AND labor.
Gotta' love Mexico. Sometimes, hate it.
Both true for anywhere else, too...

Anyway, apparently for some reason the 1500-1800rpm shudder was just the flutter-resonance induced by the specific volume of air escaping through the exh man gasket-breach at that particular amount of throttle.

Thanks for folks' input counseling a Cummins newbie.

Moral: Weird Vibration?

- Start by listening & feeling for air escaping from the exhaust manifold gasket. If none,
- THEN start messing with turbo components,
- THEN start messing with the governor springs,
- THEN start messing with the lines/connections from tank to lift-pump.

Obviously, an over-simplification.
Symptoms matter, and your case may vary.
Aloha

 

[Gypsy3000]

Update:
Arrived to pick-up truck: NO change!
Likely just a language barrier issue with Obi-Juan-Kinobe; good guy, and I trust his diesel skills.

The bov port on the exh man is already bolt-plugged. It seems likely that the valve is stuck partially-closed, restricting exhaust flow. I have no idea re the design of the stock bov inside the man; wondering if the bolt-plug may somehow be obstructing the valve from opening completely, thereby causing weirdness.

Hoping that Obi-Juan can remedy on Monday.
Will report when shudder-culprit is confirmed.

 

[Gypsy3000]

Edit: no change after EXH MAN GASKET replaced...

 

[makrop]

I was thinking at the start that I had the same exact issue, but I'm not sure after reading the whole thing. But to avoid making a new thread for my issue, did you ever try turning O/D off??? My issue is it is pretty shaky at idle, especially while letting off the brake but without adding gas. Then its ok as it gets through the gears with some lack in power. But as soon as you get to speed and shifts to O/D it shakes violently. It mostly goes away if I give it gas to cause it to downshift out of O/D. I've been driving it with O/D switched off as a "work around" for a couple months now. There is still plenty of power if I punch it and everything shifts smoothly like it should. Planning to just start with new plugs and wires, etc general tune-up. But wanted to try to rule out transmission? Any ideas from anyone or has anyone experienced this?

 

[Gypsy3000]

Pardon no update. Back wrenching this week and should report back after backing-off spring adjuster soon.

Re last reply: "gas", "plugs, wires"???
The only possible commonality with this Cummins thread would be trans and I had mine pro rebuilt with a HD Goerends torque converter. Sorry, no help here on tshooting your gasser. Advise new thread.

Related/unrelated:
On North-Central biz trip I bought an '06 3500 4x4 Cummins megacab (some rust, but great price) and then stumbled into buying a house in UP Michigan. Instead of sticking with original plan and correcting this shudder and then using this '97 as a moving-trailer tow vehicle Baja-MI, I'm (temporarily) buying a box truck for the move (Why? Because I can tow my Jeep behind the moving truck; otherwise I'd have to circus-together "'97-5thwheel-jeep" and I'm not keen on driving 2500 miles with that stress). So, after I resolve this notch-shudder I'm selling this '97 pronto. Otherwise, it runs like a racehorse. Last year of mech injection, ext cab 2500 4x4, CA vehicle (no rust). All HD trans rebuild with under 1k miles. However, the ENTIRE truck is black rhino-coated! P. O. did it but I like it. Haven't yet researched prevailing market value, but I'll extend some flexibility. I'd consider delivering anywhere in CA, NV or AZ.
[Also going to be selling a yet-unknown rust-free 16'-24' diesel box truck w/liftgate in October for under $10k, deliverable Minneapolis/Madison/Milwaukee/Chicago/Detroit]
Aloha