I have a 1990 Isuzu trooper 2.8 v6 chevy engine. I have had this vehicle for almost a year. I really like it but i cant handle the gas anymore. Its giving me 7 miles a gallon. i dont work far from my house only half mile so it been ok but i really need to fix this problem. I have spent alot of money in it trying to fix it and no luck.This is a list of what i have changed. Timing chain, air filter, fuel filter, fuel pump, pressure regulating value, injectors, IAC, spark plugs, spark plug wires, timing spark, cleaned the egr, checked the map sensor - this i tried two of my friends ones and it did nothing on the fuel so its not that. Flush out the fuel line. Checked the brakes that works fine you can push it on a flat. No codes on my truck. What i just changed about a week ago was the temperature sensor near the termostat and also i found out it had no termostat so i put one it. The vehicle reaches near quart on the gauge it was always at cold and i changed the pcv valve.My truck 02 sensor wire harness was cut off and it was reading a code that it was the wiring so i connected it back and the code went away. As you can see i have done alot on this truck and i have ran out of options. it was giving 5mpg but the termostat pcv valve and coolant temperature sensor made me go up to 7 mpg. This is still crappy miles for this truck. it has a little black smoke coming out when i rev it up also the vehicle idle is always fluctuating. Sometimes it goes at 1300 rpm and the gauge is never just one spot always going up or down. Once you drive it it runs good. So that the symptoms of it. Plz help i have spent too much on it to just let it go.
Why not do tests and end all that GUE$$ing, ok? please?
here is the list of key tests
ill skip the words TUNEUP' spark and filters...old as ages.
2wd or 4wd.?
NOT TRANSMISSION STATED, STICK OR SLUSH PUMP.
MPG DIFFERENT.!
15MPG CITY 17HIWAY, SO 7 IS 1/2 SURE,
Tests, all.(on topic)
1: engine reaches 180F and holds, there or more
no scantools in 1990 work (over the counter)
so we use an IR thermal gun, for $15whopping bucks
180f, is normal 150 is doing to suck gas like a pig, on all EFI care made.
2 sure map sensor. but a voltmeter tests it easy, the service manual shows that.
3: fuel pressure is next, i see you skipped the most important step,
if the FPR fails fuel pressure goes why high and MPG will be 1/2 normal or worse. fuel pressure at idle and gunned.
must be at spec,.
4: bad ECT, reads 150f with coolant at 180f. sure will suck fuel like mad.for 100% SURE> SM shows proper OHMS hot. RTM
5: unplugging the 02 will make mpg better, if the 02 is bad.
see why here, ill just say it...
if o2 shorts to 0vdc , the ECU goes full rich on fuel.!!!
if you then unplug it the ecu pulls that line to 0.45v and MPG is OK. if a new 02 even better even 19mpg....
at 100k miles the front 02 is changed out.
that is its full design spec'd LIFE SPAN
so it too is a tune up part, 100k miles chart.
1990 suck more fuel with bad 02
than 1996,+ USA due to OBD2 technology
OBD1 did that back long long ago.
27 long years ago,
we dont ever see cars here that old running. down south
mine sure, but others not SO.
If you had a real scan tool all this gets10x more easy
old "Tech1", or ? are hard to find and are expensive.
some shops still have one, but most are wrecked now 27 years old
Ok I'm no expert but here's my take on this. I think you solid a whole lot in this car, i had a car that had an issue with the revs going up and down she idling and would be ok when driving, that I believe is a MAF sensor symptom... I believe it should throw a code for that.
Another thing you can do is clean air intake, clean/replace idle control valve and clean throttle body, if your tb is electronic (not sure on your make and model) make sure you do the idle re-learning. That can help with the mpg a little. Recommended tire pressure also can help with mpg.
Last thing I can think of, check well the smoke that comes out, when smoke is black is burning oil (seals get worn out and overnight oil might leak in places that later will get hot), if smoke is white it's burning water (blown head gasket) and if it's blue, then it's burning gas, if that's the case, that's where your gas is going and it could relate to a failing MAF.
SOURCE: Oxygen Sensor on '99 Isuzu Trooper
The O2 sensor is on the engine side (up stream) before the catalytic converter in the exhaust pipe, it is barrel shaped about the size of a jumbo magic marker.
SOURCE: codes 171 and 174 2001 ford winstar
dont take soo much in consideration after you have make all this things, some time is on only for any wire faulty, or a shrt electrical arc in any place in wire system. ask auto tap mechsnics for this maybe they have similar case before.
SOURCE: where is the egr valve for a 1999 isuzu trooper?
The EGR valve is a cylindrical unit above the engine. It would be behind the
throttle body.
It will have a cable connected to it. Unplug the cable.
There two bolts. Simply remove.
An that is all there is to it.
Simply put the new EGR valve in its place, screw in the bolts and reconnect
the cable.
Make sure to see a gasket. The gasket can stick to either the EGR valve or
the throttle body. It does come off easily but since it is metallic, it looks
like it part of the throttle/egr valve. 
Thanks for using FixYa - a FixYa rating is appreciated for answering your FREE question.
SOURCE: where is the location of the egr valve on a 2001
on petrol V6 engine its right rear side of inlet manifold(left side when looking into engine bay) see pics. below
diesel its on inlet manifold left side middle section of manifold
SOURCE: How to clean EGR valve passages in 1993 Isuzu Trooper DOHC V6
You have to the EGR valve (sometimes the Intake manifold to EGR Vlalve line also). After you remove it take a drill bit the about 20 % smaller than the actual hole of the outlet side and rotate the drill bit with your hand to dig out the carbon. Aditionally the line coming from the intake manifold to the EGR valve will clog near the connection to the EGR valve. I use either a single piece of copper wire or the wire that you would use to tie ribar together to clean the line. When your done cleaning take a piece of vacuum line and connect it to the diaphram side and **** on it... then plug the line well with your thumb. Grasp the valve and blow thru it... Air should blow thru it freely. Remove your thumb from the vacuum line and try blowing again.. No air should pass thru the valve.
Then reinstall the valve and start the vehicle with the vacuum line disconnected (but blocked off) Use another piece of vacuum line connected to the diaphram again and **** on the line.. The vehicle should die or run really bad. If it does this then you cleaned it correctly.
slave to fork lever to above damage, at T/o bearing
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