4ZE1 idle behavior - what is normal?

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4ZE1 idle behavior - what is normal?

Postby Zenit » Tue Apr 25, 2017 5:43 pm

I have a 1990 Trooper with 2.6 4ZE1 engine. It came to me with serious idle problems - variable, surging, alternating idle at various time points. I replaced most vacuum hoses, cleaned up the throttle body and at this point the idle is at least consistent. However, I am not sure if it is normal, because I have never had a baseline to compare it to.

Right now it behaves the following way:

Cold startup: goes straight to 1500-2000 rpm until the engine is warm (the coolant temperature gauge start climbing the idle goes down). 1500 and 2000 part seems random at best.

Warm engine: Idles around 900-1000 when standing still. When car is moving in neutral or with the clutch in, the idle seems to be around 1100-1200, but goes straight to 900-100 when the car is stationary (this seem weird, never had another car that did that). This drop in idle happens when the engine is cold as well, albeit with the higher baseline (1500-2000).

Is this normal behavior or do I have more stuff to fix still?

Also, are these engines supposed to be butter smooth at idle? Mine seems a bit rough compared to other cars I've had (although I've only owned/driven subarus and BMWs before).
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Re: 4ZE1 idle behavior - what is normal?

Postby itsmehb » Tue Apr 25, 2017 8:24 pm

Probably one of the most posted of all the 2.6 problems. I know mine never idled ,what I considered correctly. I eventually just put on a weber conversion, and now no idle issues. For the itec fuel injection system to perform as it was designed to do everything has to be functioning as it was intended. The ecm controls ignition timing and fuel injector pulses in response to information provided by various sensors (map,engine temp, crank position and lots of vacuum lines that dry rot. If I still had the itec, first thing I would do is educate myself as best I could so I would understand the various sensors and how and why they function as they do. I think a lot of members here are troubleshooting the system in the dark. Idle issues with the itec system is one of the hardest issues, at least it was for me.
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Re: 4ZE1 idle behavior - what is normal?

Postby DSUZU » Tue Apr 25, 2017 10:02 pm

If the vehicle is moving, a sensor in the speedometer causes the high idle (same one that activates for A/C being on) to kick the idle up. Personally, I hate this feature, but apparently, there is no way to eliminate it without affecting the A/c idle up. It is an emission reduction feature that keeps from having high vacuum in the intake and related emissions (early carbed engines had a "vacuum release valve" that kept high vacuum conditions down). As one who likes to kick it into neutral and coast to the next light, this greatly irritates me.
Another thing that has been found is one of the 2 butterflies sometimes doesn't close all the way. Supposedly, there is a spring or tab that can be bent to help this, but I haven't looked into it yet (I'm working toward the round throttle body conversion for mine). Dennis
1991 LS Trooper 2.8 V6 5 speed conversion (Sold)
1989 LS Spacecab 2.6 5 speed 4x4 manual hubs, Rodeo torsion bars, extended shackles, 2" lift, K&N airfilter (drop in), dump bed conversion, burgundy interior change, warn brush bar.
Previously owned: 1987 Trooper II LS, 1989 Trooper (parts only), 1994 Trooper 3.2 Auto, 1997 Rodeo 2.6 5 speed, 1993 shortbed pickup 2.3 (project - sold) 1992 Rodeo 4x4 LS (parts car, devoured by Spacecab needs) plus some 70 plus other non Isuzu vehicles over the years.

http://forum.planetisuzoo.com/viewtopic.php?f=15&t=64060

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Re: 4ZE1 idle behavior - what is normal?

Postby chuffer » Tue Apr 25, 2017 11:00 pm

Zenit wrote:I have a 1990 Trooper with 2.6 4ZE1 engine. It came to me with serious idle problems - variable, surging, alternating idle at various time points. I replaced most vacuum hoses, cleaned up the throttle body and at this point the idle is at least consistent. However, I am not sure if it is normal, because I have never had a baseline to compare it to.

Right now it behaves the following way:

Cold startup: goes straight to 1500-2000 rpm until the engine is warm (the coolant temperature gauge start climbing the idle goes down). 1500 and 2000 part seems random at best.

Warm engine: Idles around 900-1000 when standing still. When car is moving in neutral or with the clutch in, the idle seems to be around 1100-1200, but goes straight to 900-100 when the car is stationary (this seem weird, never had another car that did that). This drop in idle happens when the engine is cold as well, albeit with the higher baseline (1500-2000).

Is this normal behavior or do I have more stuff to fix still?

Also, are these engines supposed to be butter smooth at idle? Mine seems a bit rough compared to other cars I've had (although I've only owned/driven subarus and BMWs before).


Sounds almost exactly like my 4ze1, which may or may not be good.

It should not be rocking or sound like it is missing. Mine dithers at idle a little and you prolly couldn't balance a glass of bubbly on the oil cap when it 's running, but I wouldn't say it runs rough...

It sounds like a lame fix, and it some ways it kinda is one, but a little liquid wrench or PB blaster down the hose that leads to the AIR valve goes a long way to helping the idle.
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Re: 4ZE1 idle behavior - what is normal?

Postby squatch » Thu Apr 27, 2017 3:13 pm

OP it's not to far from normal! Most have a little variance even when tuned well. Same with slight wandering of 100 rpm or so at rest.

Dennis the 2 butterfly's in the Throttle Body are supposed to open at slightly different rates. It's to keep down tip in bog.
These links have write-ups and pics of most anything you ever will do on a 2.6 Isuzu. Engine, brakes, suspension, interior, you name it! Enjoy!
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Re: 4ZE1 idle behavior - what is normal?

Postby Zenit » Thu Apr 27, 2017 4:46 pm

From these replies and looking at the diagram in the shop manual I think mine does act OK.

The thing that was throwing me off is that increase in rpm when car is moving, even while rolling in neutral. The itec system does appear to have an input of vehicle speed in the computer that goes to air/fuel mixture calculation.

As for vibration, could this be the reason (see pic). It appears that the motor mount may not be good on the passenger side (see the big rough edged hole on top of the image). Anybody have a video on how to replace them?
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Re: 4ZE1 idle behavior - what is normal?

Postby DSUZU » Fri Apr 28, 2017 5:58 am

Have you checked compression on the engine? Also valve adjustment? These can also cause vibration. Dennis
1991 LS Trooper 2.8 V6 5 speed conversion (Sold)
1989 LS Spacecab 2.6 5 speed 4x4 manual hubs, Rodeo torsion bars, extended shackles, 2" lift, K&N airfilter (drop in), dump bed conversion, burgundy interior change, warn brush bar.
Previously owned: 1987 Trooper II LS, 1989 Trooper (parts only), 1994 Trooper 3.2 Auto, 1997 Rodeo 2.6 5 speed, 1993 shortbed pickup 2.3 (project - sold) 1992 Rodeo 4x4 LS (parts car, devoured by Spacecab needs) plus some 70 plus other non Isuzu vehicles over the years.

http://forum.planetisuzoo.com/viewtopic.php?f=15&t=64060

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If your wife has 5.00, and you have 20.00, your wife has...25.00

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Re: 4ZE1 idle behavior - what is normal?

Postby squatch » Fri Apr 28, 2017 2:45 pm

That's the torque side mount. Looks like time for new ones. PM Jlemond on this site for them. They don't cause vibrations. But can really let them show when the mounts are beat.

The job is not hard. Just lift the engine a little buy the ring near the power steering pump and radiator hose. Un bolt the old ones and install the new ones. I think there is pics in my links below.
These links have write-ups and pics of most anything you ever will do on a 2.6 Isuzu. Engine, brakes, suspension, interior, you name it! Enjoy!
Original '88 Trooper frame off build.
viewtopic.php?t=12172
The continuing Trooper project.
viewtopic.php?f=15&t=43909
My '88 Spacecab build. In progress.
viewtopic.php?f=15&t=56442
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Re: 4ZE1 idle behavior - what is normal?

Postby Zenit » Sat Apr 29, 2017 5:32 pm

Don't have anything to lift the engine with. Will jack and a piece of wood under oil pan work? Or too dicey? Never replaced motor mounts before.

Also, are aftermarket motor mounts such as DEA, Westar and Eagle BHP worth installing? They don't seem to be overly expensive.
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Re: 4ZE1 idle behavior - what is normal?

Postby DSUZU » Sat Apr 29, 2017 9:51 pm

I replaced mine using a floor jack and a block of wood. Try to undo as many of the mount bolts (all?) as possible before raising so that time in air is limited. Page 34 of my build thread covers replacing mounts on my Spacecab pickup. Note that my front diff is removed making it much easier. Dennis
1991 LS Trooper 2.8 V6 5 speed conversion (Sold)
1989 LS Spacecab 2.6 5 speed 4x4 manual hubs, Rodeo torsion bars, extended shackles, 2" lift, K&N airfilter (drop in), dump bed conversion, burgundy interior change, warn brush bar.
Previously owned: 1987 Trooper II LS, 1989 Trooper (parts only), 1994 Trooper 3.2 Auto, 1997 Rodeo 2.6 5 speed, 1993 shortbed pickup 2.3 (project - sold) 1992 Rodeo 4x4 LS (parts car, devoured by Spacecab needs) plus some 70 plus other non Isuzu vehicles over the years.

http://forum.planetisuzoo.com/viewtopic.php?f=15&t=64060

The American dream is getting farther and farther away from the American.

If your wife has 5.00, and you have 20.00, your wife has...25.00

"Water is one of the greatest essentials of life. Without water, you can't make coffee."
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Re: 4ZE1 idle behavior - what is normal?

Postby Mrakulous » Sun Apr 30, 2017 11:08 pm

Mine runs similar. New head and gasket and new vacuum lines. Cold Rome at 1100-1200. Once warm idles at 900. Will god up to 1100 when I stop. If I feather the throttle it goes down. Runs great though. Get 18 mpg consistelant. I'm not too worried. It doesn't miss or surge. I had the air temp sensor loose that caused running problems.
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Re: 4ZE1 idle behavior - what is normal?

Postby Zenit » Tue May 09, 2017 11:45 am

Well it looks more and more that the idling behavior I am getting is normal. Did another vacuum leak test with propane - did not find any, also did a compression check and it looks fine as well.

Cyl 1: 155
Cyl 2: 160
Cyl 3: 145
Cyl 4: 160

Did a wet test on 3 and 2 for control and got following:
Cyl 3: 160
Cyl 2: 170
Both went up 10-15 with oil added. Do I have some kind of valve problem with #3?
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Re: 4ZE1 idle behavior - what is normal?

Postby DSUZU » Tue May 09, 2017 4:08 pm

Increase by adding oil is usually a sign of worn rings. If you add a bit too much oil, the space taken by the oil increases compression pressure. Usually about a tablespoon and crank it over good to spread. Dennis
1991 LS Trooper 2.8 V6 5 speed conversion (Sold)
1989 LS Spacecab 2.6 5 speed 4x4 manual hubs, Rodeo torsion bars, extended shackles, 2" lift, K&N airfilter (drop in), dump bed conversion, burgundy interior change, warn brush bar.
Previously owned: 1987 Trooper II LS, 1989 Trooper (parts only), 1994 Trooper 3.2 Auto, 1997 Rodeo 2.6 5 speed, 1993 shortbed pickup 2.3 (project - sold) 1992 Rodeo 4x4 LS (parts car, devoured by Spacecab needs) plus some 70 plus other non Isuzu vehicles over the years.

http://forum.planetisuzoo.com/viewtopic.php?f=15&t=64060

The American dream is getting farther and farther away from the American.

If your wife has 5.00, and you have 20.00, your wife has...25.00

"Water is one of the greatest essentials of life. Without water, you can't make coffee."
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