1HDT idle fluctuating between 800-1000rpm

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Sep 30, 2014
Hey guys,

Recently purchased a 91 cruiser with a 1HDT with a manual transmission.

Noticed yesterday when starting that the idle would sit on 800rpm (nice and smooth) but every say 3 seconds it would jump up to 1000pm before dropping back to 800rpm. It’s almost like someone is blipping the throttle...

It was very consistent for about 3 minutes until the engine warmed up. Was the first “cool” day I started the engine (was 14 degrees C) and noticed this, previously it’s been 20+ degrees.

Physically looking at the pump I don’t see any movement in linkages at all...

Any ideas what to look at? This is my first 1HDT. It’s done 250,000km and has been regularly serviced.

Might be the governor
AC idle up. Next time it happens, hit the AC button and see if the idle drops back down.
Thanks guys,

So the A/C itself doesn't seem to work, I think its had a leak/out of gas (the compressor clutch engages if i apply 12v directly).

I did check and the A/C was switched off.

So a little more info: I just pulled the glow plugs out and did a compression test. (Engine was warm during test)

Cylinder 1 = 480psi
Cylinder 2 = 470psi
Cylinder 3 = 475psi
Cylinder 4 = 445psi
Cylinder 5 = 480psi
Cylinder 6 = 480psi

So obviously cylinder 4 is a little low compared to the others but still pretty reasonable. Note: I haven't checked valve shims yet, thats next on the list.

One thing i did note was cylinder 1-4 glow plugs were a nice healthy dry brown colour, but glow plugs 5 and 6 were both black and very wet...

So it seems to be blowing a grey smoke (It appears black when it first exits the exhaust but as it dissipates it turns grey). Its defiantly not blue/smelling of oil. EDIT - So theres a slight hint of smoke at idle, a bit during revving but if i push the throttle hard there is a big cloud of smoke. So its either incorrect pump setting or maybe faulty/leaking injectors. If an injector fails is it common to inject too much diesel?

Now it has had a top mount intercooler fitted by the previous owner who apparently "tuned the pump"... So I'm wondering if its a case of a faulty pump, faulty injectors (cylinders 5 and 6) or just incorrect pump settings. Thoughts?

Another cause could be a loose fan/alternator belt that gets tight when its warmed up

So I'm wondering if its a case of a faulty pump,

The governor is inside the injection pump and I was thinking it may be sticking somehow. Try revving it above 1200+ and see it disappears faster.
If an injector fails is it common to inject too much diesel?

Not really, an injector can only inject the amount in the charge, that is the metered amount measured in cc's the pump sends to each injector.
Being a rotary pump, it has a single plunger sending fuel to all 6 injectors in turn.

I dont think its a serious problem because it goes away as soon as its warm. Look for videos on youtube about how diesel governors work. Im thinking the governor may have a tired spring.

This is a good video although the fuel pump is a bit different, the principles are the same. The 1HD T has been shown in the past to have a fairly delicate spring and many have been affected by pointing the nose uphill which can cause them to idle lower and stall.
I wouldnt start tearing the pump apart just yet. It could be something more simple.
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yep i had one from brand new and it was quite bad along with surging went back to the dealer quite a few times in the day they finally fixed it but never said how it was a very common problem i was very disappointed after my 12 ht which i went back to....
Mine does the same, '91 hdj81. Anyone know what needs to be changed in the IP? I just did a reseal, leaking shaft seal, was hoping it would also fix the surge but didn't.
Hunting for idle usually is associated with timing. Maybe it’s the ACSD, need to delete it anyway. If it still does it after the delete, check the timing
ACSD has been removed on mine and timing set to 1.35 a few years ago. The idle hunting didn't change after removal. I've never got around to messing with it more since if I keep the idle speed at 750 and ac up to 800 it doesn't surge. If I go a little higher it shoots to 950. Doesn't bother me too much but the OP got me thinking about it again.
Have a read here

The guy that posted this seemed to be a very knowledgeable fella, and diesel tech.
Haven't seen him post in ages.

From reading that post, sounds like the solution requires a clued up Denso repair tech, and a fist full of dead Presidents

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