I have a 1988 Isuzu Trooper II wih 2.6 Liter 4 Cylinder Fuel Injected engine. I'm going to try an pack alll the info here that I can , hopefully understandable. WHehen I purchased the truck used it purred like a kitten and had plenty power. No problems whatso ever. Then the problem started. It still idles without missing a beat and will sit all day long ideling perfect. I have read and surched this forum until I can read no more, So I will try this and hopefully some one will have run into a similar issue.

1> It idles perfectly but has absolutely no low end power below 3000 rpm.
If ou get it above 3000 rpm it has more power but not as much as use to have. It will miss (no backfires) seems like it's starving for gas. The colder the ambient temperature outside, the problem is worse. If it gets down around 15 degree, it will still idle without a miss but it seems to have very lowe power no matter what the rpm.

a. I have changed the fuel pump and screen.
b. I have replaced the filter numerous times.
My Nephew is one of the best mechanics around and has all the computerized troubleshootin tools , but has found no problem diagnosing it from that angle.
c. We have changed the plugs, Distributor cap and wires.
d. I have replaced all the vacuum hoses, one at a time to make sure none were connected incorrectly ( assuming they were correct initially , since it ran perfect for so long). I did find numerous vacuum hoses split where they slid over their connectors, no improved .
e. We replaced the mass air flow sensor.
f. We checked the fuel pressure past the fuel pressure regulator and it was correct.
g. the O2 sensor was replaced.
h. Air filter was replaced.
i. it uses no coolent and absulutely no oil (amazing) never had a vehicle that used no oil at all, especially at 141,000 miles..LOL
j. I get no check engine light. So no codes.
k. Water temp runs normally
l. Have run feul injector and Hi octain fule in it with no improvement (grasping at straws)

2. I did notice in the FSM that thre is a Fuel Temperature sensor I have not replaced. The closest Isuzu dealer is 3 hours away.

3. What bothers me is the greater loss of power as the outside ambient temperature drops. I have not a clue what to check next. O I have had to add some heet because there have been some condensate in the gas tank because of 75 degree days adn 35 degree nights. I have also cleaned the gas tank out.

If anyone has a clue, please pass it on to me.

Thanks
Ruben

Continuing to search the forums also.

 

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My bet is you've got a blown head gasket. Have your nephew do a compression test on it and I'll bet you get low numbers. These engines are known for bad head gaskets and very often do not show the typical symptoms like water in the oil.

Please get my drift here:
https://isuzuforums.com/forum/showthread.php?t=39243