HZJ78 hiccups/hesitation above 2000 rpm

[janyyc]

Went for a quick camping trip last night and started noticing a hesitation above 2000 rpm, especially under load (e.g. uphill). Every gear the same. My instinct is air or fuel, so fuel filters will be replaced first tomorrow. Is there anything else that could be causing this that I should look into before I leave again tomorrow?

Detailed description: up to 2000 rpm, very smooth, no matter the load. Above 2000 rpm, starting at about 2200 rpm, there are really small, quick hiccups. RPM stays steady, but the hiccup is both heard (engine tone, ever so briefly) and felt (a small backing away). But the pull then resumes as if uninterrupted. The best way to describe is that it feels like backing off the accelerator very very briefly (almost superhuman short), only I'm not.

What I've checked:
- air filter (clean)
- pressure inside fuel tanks (both fine, lots of altitude changes here but no pressure when opening cap)
- happens on both sub and main tank
- happens on every gear, only above 2000 rpm
- rpm will continue to increase without issue, hiccups possibly more pronounced closer to 3000 rpm
- this is a new issue, and nothing had been worked on or changed in the lead-up to it appearing

Furthermore, I doubt it's the injectors or IP, since engine has 70K km on it. Also, there's no EGR on this 1HZ.

 

[Rigster]

Hey Jan,

I can't remember what year your troopy is... but does it have the altitude compensator built into the fuel pump? If so, double check the vacuum lines, routing, make sure nothing is loose, or has any leaks. A vacuum tester works best, as you will never hear it, though sometimes you can get lucky and see where a hose is wearing through, or cracked. To test the actual valve, you will need a vacuum tester.

You are correct in it will be either fuel or air... that is all these run on!!

 

[roscoFJ73]

I doubt it's the injectors or IP

It sounds like some kind of fuel starvation, so I wouldn't discount the injection pump just yet. I would rig up a temporary fuel supply to isolate it between fuel supply and fuel delivery.
Ive felt this before on 1HZ's Ive owned but more when I feathered the accelerator on hills giving it just enough fuel , but it wasnt as pronounced or as regular as what you have described.

 

[Rigster]

Agreed, I am pretty sure it is fuel related, and to do with the boost compensator with the governor. It is exactly right at the "power band" (for lack of a better term) spot just over 2000rpm when these engines seem to really generate their power. It is like it is sticky or something in that transition.... my 2 cents, thinking out loud...

 

[janyyc]

Fuel filters replaced yesterday and problem is completely gone on the drive to the airport!
Thanks everybody

 

[Rigster]

Fuel filters replaced yesterday and problem is completely gone on the drive to the airport!
Thanks everybody

Glad you got it sorted!