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Discussion Starter #1
Thanks Erik.

I've identified the aneroid boost compensator (abc) on my IP, but I'm not sure if there is a altitude thing next to it. I'll try to take a picture and post.

BTW, the hose going to the abc is dry cracked and broken. I'll fix it and see what happens. Ultimately I'd like to experiment with removing or disabling the abc and or the altitude device if it exists.
 

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From what I understand you will have one or the other, either boost compensator or altitude compensator. I have an altitude compensator because I have a factory anti tamper cap on the nipple at the rear of my altitude compensator. I have a spare laying around and I may try hooking the nipple up to boost to see how it reacts though.
 

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Discussion Starter #4
I replaced the broken/cracked vac hoses to the anerobic boost compensator, made no difference. Truck still runs the same.
 

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Thanks Erik.

I've identified the aneroid boost compensator (abc) on my IP, but I'm not sure if there is a altitude thing next to it. I'll try to take a picture and post.

BTW, the hose going to the abc is dry cracked and broken. I'll fix it and see what happens. Ultimately I'd like to experiment with removing or disabling the abc and or the altitude device if it exists.
There is no reason to remove or disable a working boost compensator, they're adjustable right down to barely working and mine is fully open as soon as it receives 4psi. Think of it as an adjustable switch that tells your fuel pump when the turbo has woken up.

Take the hose to the boost compensator and blow down it (clean it first:rasta: ). If you can hear or feel any leaks, then the diaphragm inside is possibly perished. It cost me about $US50 for a new diaphragm about 18 months ago, that turned my engine from slow to great fun instantly.

Some engines have both the boost compensator and altitude compensator, some only have one and some have neither.
My engine has the boost compensator but not the altitude compensator.
 

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Ok, so fixing the vac hose for the boost compensator did nothing, so I thought, what the hell maybe I'll turn out the fuel screw. So I did turn it out all the way completely. I noticed a tiny bit more power but I'm not impressed, and I'm not seeing any clouds of smoke. While fiddling around under there, I felt air blowing on my hand near the back of the engine. Upon closer examination I discovered that the intake gasket was not installed properly on #4 cylinder. There is a gap the thickness of the gasket between the manifold and head, and air is blowing out freely. So, clearly I have little or no boost. Lucky by chance I discovered this. I'm sure it would have been come to light if I had a boost gauge installed. (boost and egt guauges comming soon). Perhaps my truck has been running like a naturally aspirated engine this whole time. It's damn obvious considering the position of the gasket hanging out that it's was a faulty install. Probably some dumbass with a hangover on a Monday. Anyway, my theroy is that the boost compensator is working and holding back fuel because it's not getting an adequate boost signal considering my air leak. Maybe why I'm not seeing black smoke? Opinions anyone? I'm going to get a new gasket asap and report my findings.

Dougal: Re: adusting out the boost compensator. I see what looks like a locknut with wire through it on the boost compensator diaphram body. Should I just loosen that nut and back out the boost compensator diaphram body?

Thanks!
 

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Here is a pic of the back of my pump. Does it look like I just have an anerobic boost compensator? Or is there an altitude device too?

That disconnected hose you see there in front of your face connects directly to the intake manifold. (Boost signal I'm sure)

I know this pic is crappy. There are so many things in the way. I can try to get a better picture tommorow.
 

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It's hard to see from the photo, but you may have both altitude and boost compensators.

But yes, with no boost your boost compensator will be the fuel stop and the max fuel screw will do close to nothing.
The adjustment for your boost compensator is actually on the rocker link, there's a plug around 16mm in diamter, under that is a 6mm screw and 10mm locknut.
But if it's working properly, there's no reason to adjust it. Fix the intake leak and see what you've got.

With no boost, the boost compensator is probably the only reason you haven't melted a piston.
 

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Discussion Starter #9
So does low boost & too much fuel = high egt's? Is that where the piston melt risk comes from? Is this why we need to watch our egt's?
 

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Discussion Starter #10 (Edited)
Ok, I've done some reading of my shop manual. I do not have an altitude compensator. There are 2 devices on the back of my pump. One is the aneroid compensator. The other is the boost compensator. They are side by side, connected integrally and work together internally via a common lever. The broken/dry cracked vacume line I mentioned earlier in this thread is for Fuel Low Cut function. This is activated if the engine coolant temp exceeds 208 degrees F. The fuel low cut function utilizes the aneroid compensator mechanism by means of a diaphram. Basically if the engine gets to hot, the fuel low cut function cut's back fuel via the aneroid compensator, in much the same way as if there was not enough boost. Thats why there are 2 lines connected to the back of the injection pump. One is vacume as I just mentioned. The other is boost directly from the intake manifold.


So in effect there are 2 separate diaphrams. One is the aneroid compensator- vacume actuated. The fuel low cut function is activated by vacume via an engine coolant sensor which in turn activates vacume valves which in turn actuates the aneroid compensator diaphram.

The boost compensator diaphram works like a piston - the force of boost pressure moves the piston thus increasing or decreasing fuel relative to boost.

So to summarize, there are two mechanical compensator devices (aneroid compensator and boost compensator) each has it's own separate diaphram actuator (1) (aneroid) vacume (fuel low cut function to prevent overheat/cuts back fuel relative to excessive engine coolant temps) and (2) (boost/pressure) (which can cut back or increase fuel relative to boost).

My shop manual actually has some very good diagrams which illustrate the above. I wish I had a scanner to post it here, but alas I dont.

All this considered, I believe my boost compensator is working correctly as dougal suggested.

Lucky me, my isuzu dealer is 2 miles away and they have my intake gasket in stock. I'll change this out probably this weekend.
 

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... so I thought, what the hell maybe I'll turn out the fuel screw. So I did turn it out all the way completely. ...
In normal circumstances, that action would most likely cause your engine to destruct itself. Either revs run-away, or melt pistons as Dougal indicated.

You should return the screw adjustment to where it was, then find and fix the underlying problem.

Once that is fixed, and preferably when you have boost and egt gauges, adjust the max fuel screw by about 1/4 turn at a time between road/load tests.
 

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FWIW the intake gasket P/N you should need is in the P/N stickey thread Dougal started at the top of this forum.

Yeah, I have the same setup.

I have not gotten my new (to me) turbo screwed to the engine yet, but I have all the stuff to put a pyro and drive pressure tap in (I put them in the spacer I had to make for the larger compressor housing to clear the manifold :D ).

Have to see how that turns out when the engine should give full fuel earlier (I'm expecting an earlier spoolup out of the HY35 as the shaft will be turning as opposed to randomly rattling around inside the bore :rolleyes:).
 

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Ok, I've done some reading of my shop manual. I do not have an altitude compensator. There are 2 devices on the back of my pump. One is the aneroid compensator. The other is the boost compensator.....
The aneroid compensator is the altitude compensator. Its primary purpose is to reduce fueling as the altitude increases and the air gets thinner. Isuzu just got clever and also used it to cut fuel when the engine gets overly hot. If you notice in your diagram, when the engine is at normal temperature, the aneroid (altitude) compensator port is open to the atmosphere so it modifies fueling based on air pressure. When the water temp goes too high, Isuzu simulates a higher altitude by applying a regulated vacuum from the engine's vacuum pump making the IP think that it is operating at a higher altitude than it really is and therefore reducing the amount of fuel. On the 4BD1's, there was no temp compensation so the port on aneroid compensator was just open to the atmosphere.

Below is the diagram that I think you are referring to. Notice that when Valve #2 is NOT activated by the Temp switch #6, the output port is connected to the atmosphere. I believe that the the square box with the X in it on the valve represents the little foam filter that is on the end of the valve.
 

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Discussion Starter #18
Great info!

Here are a few more pictures:
 

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There are some mistakes in the 2nd pic- 'Figure 3.

One way to make the item arrows (call-outs) on the diagram correspond with the item list is:

Delete the arrow that points to the max fuel screw and the number (18) .

Change the number call-out (19) on the arrow that points to the control rack (top-left) to (18) .
 
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