1989 Isuzu Amigo Driving Me Nuts!

[Joe In Montana]

Vehicle:

​

• 1989 Isuzu Amigo, 2.6L FI, manual trans, AC, PS, 2WD

Symptoms:

• Starts cold and runs great until..... until the ECM takes over and goes into Closed Loop.
  
• Then it starts missing, farting around, no matter where the throttle is or the road speed or the engine RPM.
  
• There are times it runs perfectly - in a small band of the right RPM - load, gear ratio (manual 5-speed).
• The 'correct band' where it operates best is anywhere it feels like it wants to be.
  
• Cold idle vac. control is now acting a little strange at decel coming to a stop, as it used to hold 1300 RPM until full stop then drop to 900 RPM.
  
• Now the Hold-Till-Stopped RPM is 2000, then 900 as usual.
• Altitude has no change in performance - I just went to Big Hole and back to 3000 feet (home) @ 200 miles round trip today and the problem is always there - mostly.
  
• 70MPH is a good speed with a minimum of missing and bucking - up hill or down, with or against the wind.
• 55 MPH works pretty good - not a TPS or road speed problem - I think.
  
• RPM seems to make no difference.
• Engine load MIGHT be somewhat affecting the problem. Or not.
  
• I CAN bring the symptoms on and if I 'play' with the throttle, I can almost make it go away too.
  
• If I turn the heater ON - it seems to get a little better.
• There are NO engine/ECM codes and no CEL.

Done So Far:

• Tested KVs on all wire near plugs = 3.5-4.6 KV, Idle. Coil wire = 4.8 KV/Idle
• Snap throttle - plugs = 6.6 KV (no running load test done)
  
• R&R plugs - no dis-colorization, all look good - NGK Dual Plats with 18K miles.
  
• Opened distributor - clean, new cap and rotor.
  
• I disconnected the Oxygen Sensor - no difference.
  
• Fuel pressure test - within normal limits. Removed vac line to FPR to test - no difference.
  
• Timing is 6BTDC - which is correct for this engine as the outer balancer ring moves.
  
• Fuel has always been 87 R+M/2 Octane.
  
• Ohm meter test on TPS is smooth, no bad spots.
• CTS is operating as normal for all engine temps.
  
• Engine oil is up to the mark
• Coolant level is full, thermostat is working at 180F.
  
• Vacuum leaks at high engine RPM won't make much difference if there were any - but at idle there are NO leaks anyway.
  
• Muffler and cat are free flowing.
• Air filter is clean-ish.
  
• EGR works correctly
• Valve adjustment was OK - no broken valve springs either.
• Cleaned MAFS - no difference

What I think I'm going to do next:

• Exhaust manifold has a leak that in the past - never caused any trouble - and I have a new manifold to install - I doubt that an exhaust leak after the Ox Sensor is problematic.
  
• This engine eats exhaust manifolds every three years or so. I have extras.
  
• There is no Post Cat Sensor - this is an antique Pre- OBD2 system! (remember blinking lights?)

Notes:

• Engine seems to have good power when it's running right.
• Like I say: cold it is perfect - once it goes into Closed Loop, all hell breaks out.
• Fuel mileage is still 22+
• There is NO backfiring.
  
• There is NO belching in the intake system either.
  
• It never 'seems to want to die' - just run badly.
• It starts very easily - every time.
  
• It will idle for days, waiting to go - if I want it to.
  
• Cooling fan cycles at 220F - ON / 180F - OFF, (my own installation of a 1960 Opel electric fan and a Honda CVCC temp sending unit).
  
• There is no mechanical fan on the water pump any more.
  
• I don't fear that it can't bring me home when I take it trout fishing. I hope.

Where I am right now:

• Like I say: 'It's vague"
• Anyone got any ideas?
• Lost.

 

[K5wrench]

Connect a vacuum gauge to a manifold vacuum source.
Route the line into the cab and drive it around a while.

-weak valve springs?
-cam lobe going flat?

I had an Opel GT with over 300k miles on it.
Ran perfect.
Did 115 mph on the highway (i was in highschool then)

One day, it began missing.
Very much the symptoms you described.
Though it had no closed loop mode or computer, just a 2 bbl solex carb.

No matter what i did, could not get it back in tune.
Vacuum test showed erratic activity.
Pulled the head and had a valve job done.
Found several cam lobes were worn flat.
No bump at all visible.
Engine shop found me a cam.
Ground my valves.
That little car went like stink again.

 

[K5wrench]

One more..
Dirty egr passage or sticking egr valve.
On many of those the EGR is deativated during open loop mode.
Maybe yours hangs open during closed loop somehow.
Again, vacuum gauge will find that.

 

[Joe In Montana]

One more..
Dirty egr passage or sticking egr valve.
On many of those the EGR is deativated during open loop mode.
Maybe yours hangs open during closed loop somehow.
Again, vacuum gauge will find that.

The EGR uses a heat-sensor to make sure that it is working and driving heat up in the EGR circuit at the right time. Kinda Fred Flinstone, but it is very sensitive. I know it opens - but I'm not sure it closes when it's supposed to.

I'll check that out.

I just yesterday was into the valve cover and adjusted the valves to see it there was something wrong in there.

No broken springs, no flat lobes and no bent or missing followers (OHV). I worry about that because of the NO ZDDP in the oil nowadays.

Weak springs? Maybe. I'll recheck that tomorrow. Will also run a vacuum test.

Thanks - hopefully this'll find something!

 

[K5wrench]

I remember my Dad bringing home an Izuzu P'UP one time.
He'd do things like that; trade a car in without warning and buy something else.
Drove my Mom nuts.
She'd never put a full tank of gas in her car in case he sold it suddenly.

Anyways, my Dad parks this silver Izuzu pickup in the garage and starts tinkering on it.
Next thing you know, there is a horrible screeching sound out of the attached garage.
Mom screams as she opens the door.
A steady stream of sparks are flying off the roof of the new truck as my Dad uses a circular saw with a fiber blade to cut a giant hole for a sun roof.
Never forget that.
Damn the old man was crazy.

That little Izuzu was a fine truck for what it was.

 

[Joe In Montana]

I remember my Dad bringing home an Izuzu P'UP one time.
He'd do things like that; trade a car in without warning and buy something else.
Drove my Mom nuts.
She'd never put a full tank of gas in her car in case he sold it suddenly.

Anyways, my Dad parks this silver Izuzu pickup in the garage and starts tinkering on it.
Next thing you know, there is a horrible screeching sound out of the attached garage.
Mom screams as she opens the door.
A steady stream of sparks are flying off the roof of the new truck as my Dad uses a circular saw with a fiber blade to cut a giant hole for a sun roof.
Never forget that.
Damn the old man was crazy.

That little Izuzu was a fine truck for what it was.

Yeah - they're tough as my wife's fried liver.

I've got almost 400K on it - 130K with a rebuilt engine that I did because it dropped the thrust bearing in the engine and ruined the block and crank.
Immediately I disconnected the neutral safety switch on the clutch so I could start the engine without pushing in the clutch.

I never liked having a dry thrust washer - a HALF-MOON thrust washer - to hold the crank in place with my foot on the clutch. Dry bearing and starting rotation = ripping the half moon out.

The transmission's like a faithful puppy - not a whimper and just keeps on going.

I painted it 1956 Porsche Bathtub Speedster Racing Red a few years ago - 5 years to be exact.



The interior looks like where Ratso Rizzo lives - well, the driver's seat does anyway. Just getting comfortable now!

I hate the monkey-motion front end suspension - or what passes for suspension. It has a total lift/drop of about 3/4" I think - on torsion bars that I surmise the torsion all leaked out by now.