Just when I think I understand this engine. I've got a high mileage truck ('89) that I tuned up about a month ago. It was running great... for a week. Good acceleration, great mileage (28 MPG hiway). Then the idle speed began to decline followed by hesitation on acceleration followed by the check engine light flashing on. This is the list of the things I've done:

Replaced cap, rotor, wires, plugs
Removed and cleaned throttle body
Replaced pcv valve
Replaced O2 sensor
Checked and cleaned EGR circuit
Reset Throttle Position Sensor
Adjusted Valves
Set timing
Replaced suspect vacuum lines

Net result: Still hesitation, still check engine light, still great mileage

The check engine code only flashes on the highway and will go off instantly when I release the throttle pedal or if I floor it, and stay off for another ten miles or so. The code indicates "O2 sensor" but seeing as it's new I think it's just registering excess or not enough oxygen in the exhaust stream. I've adjusted the idle screw all the way out and can barely muster 800rpm at idle. There's no fast idle on a cold start. If I don't tap the throttle a couple of times it just dies. Once it's warmed up it doesn't die but idles too low. To me this points to an air leak on the intake side, but I've checked and rechecked all the vacuum lines and did the flammable liquid thing on the throttle body chamber (there's got to be a less dangerous way to check gaskets) with no change in idle speed. I'm stumped. Any suggestions?
Is there a step-by-step way I can bypass elements of the vacuum circuit to try to narrow down where the problem is, or am I on the wrong track?

One of the things you've mentioned that rather stands out is your mpg.
I've always been pleased with my mpg (around 23-25) but 28? I believe you, because I once hit 28+ on a road trip that was almost all downhill, with a tailwind.

So anywho, I have a thought.
Have you pulled & checked your new plugs? Look to see if there are any signs of higher then normal temps.

You've mentioned your theory, but I'm curious if whether or not, your ECM/fuel injectors are injecting the proper amount of fuel. ie; maybe it's running too lean.

It's your high mpg, hesitation, and o2 light flashing that made me think of this.

luck

Travis

I marvel at the mpg myself 'cause it's actually improved over when it was new. Less mechanical resistance in the drive train I guess. It's relatively flat terrain for a long commute with no load and I'm no jack rabbit, so I guess that helps.

Too lean is a thought. There is a code in the book for a lean condition but it hasn't flashed, just the O2 code.

I'll pull the plugs, but then I guess that would point to a fuel supply problem? I was trying to figure out where the regulator for the fuel pump pressure in the fuel rail would be and it looks like that pneumatic diaphragm on the front side of the throttle body chamber. When I pulled the line I didn't feel any vacuum though so I'm not sure. At the least I'm going to change the gas filter and throw in a bottle of injector cleaner.

Thanks for the feedback.

Fuel supply problem is possible, but I was thinking more about the fuel/air mix. If it's running too lean, there might be evidence of this on the tips.

Wonder if your ECM is mis-reading the new o2 or mixed signals from the MAF.

Have your experienced any predetenation? Any slight pings or knocks?

As you mentioned, it would worth the effort to run some quality injector cleaner through the tank.

Travis

I changed the fuel filter out (it was clean) so supply does not appear to be the issue. Pulled the plugs, light soot coating which would point to too rich, but I think it's a temporary conditon from the injector cleaner. so I'll run it for another week and pull them again. In answer to the question, no clatter when the timing is set at 12 degrees, but it hesitates
all through the pedal range. If I set it at 14 degrees, hesitation is gone in the low range of acceleration and diminishes in the upper range although there will be a slight valve clatter initially. There has been no change in the "13" code flashing up while driving and it still disappears when I let off the gas or tromp on it. Another thing I've noticed in road tests is if I feather the pedal, back off slightly when I fell the hesitation, then press the pedal again it's as if something has reset because the hesitation is momentarily gone. How do you check the MAF?

Todd Marcucci has an excellent write up on the MAF posted on his website

Scroll about a third of the way down to read about checking the MAF.

Hmm, weird. Like you said, light soot coating (possibly rich?) yet mpg is way up. I wonder it your ECU is enriching or leaning out, out of spec at various points in the rpm range? About the only way to find that out (that I know) is to strap it on a dynometer, to get a air/fuel map ratio chart.
Interestingly enough, Todd and I discussed the pros/cons of installing an aftermarket fuel/air computer (like the Apexi S-AFC) on a 2.6l. Afterall, the sport impact crowd are religious about retuning their ecu's whenever they perform any mods; to maximize the gains. Only makes sense that we could and should do it to. But while the Apexi unit is reasonably priced, dyno time is not!

Starting to get stumped here, might need to call in the big-guns on this one.
Mike, Dan, Todd, or Gary Lemond.
Let me know what you think.

Travis

OK, I'm officially giving in and allowing my p'up to do anything it want's as long as it keeps going. After running through the injection cleaner and a couple fresh tanks of gas the roughness has smoothed out, dying on a cold start has stopped and I've picked up another half mile per gallon, but I still can't get the rpm's up and it still has the hesitation on acceleration. I'm pretty convinced it's the throttle position switch, mostly 'cause that's the only thing left that would explain the symptoms.
Being that it's a $90 item at the local dealer I'll put up with less than perfect performance until something more definite tells me I have to replace it. Thanks for the tips and feedback.

Well I quess there are worse problems in life to deal with then getting nearly 30mpg!

I know I've heard of these symptoms before, though for the life of me I don't remember where!
Luck

Travis

I am having the same issues with my Amigo. Where is the throttle position switch located?

Thanks,
Bill

It is called a throttle position sensor or TPS and it's located on the side of the throttle body. It's on the firewall side of the throttle body.