Part throttle stumble closed loop operation.

[wankel1308]

1994 5 speed 4x4 297k miles. Problem I have is a bad stumble and hesitation when in closed loop operation but only at partial throttle. If go full throttle it straightens out and pulls hard to redline. In open loop, it runs great, starts on the first crank, idles smooth, pulls at all throttle positions at all rpm. Once it warms up it falls on it's face unless you floor it.

I haven't been able to find any vacuum leaks. I'll admit it's possible that it just has a bunch of tiny ones.... Or I'm just being bullheaded and missed one. The intake gaskets have been recently replaced, but I did not open the injector rail.

Most of the engine sensors for timing and fuel control have been replaced. The IAC has not been, but it makes no difference if it's unplugged or not. Throttle position sensor is new, MAF is new and clean, ECM is new, crank position sensor is new. Plugs and wires are new. Fuel pump is new as is the pressure regulator. DPFE sensor is new and differential pressure high is the only code the truck will throw.

Am I looking at an O2 sensor issue? No codes for it, but maybe? Should I just buy an IAC? Is it a stupid vacuum leak? Just throwing ideas at the wall here somewhat, also trying to build a paper trail for anyone else who might have this particular failure mode. Thanks!

 

[Fix4Dirt]

1994 5 speed 4x4 297k miles. Problem I have is a bad stumble and hesitation when in closed loop operation but only at partial throttle. If go full throttle it straightens out and pulls hard to redline. In open loop, it runs great, starts on the first crank, idles smooth, pulls at all throttle positions at all rpm. Once it warms up it falls on it's face unless you floor it.

I haven't been able to find any vacuum leaks. I'll admit it's possible that it just has a bunch of tiny ones.... Or I'm just being bullheaded and missed one. The intake gaskets have been recently replaced, but I did not open the injector rail.

Most of the engine sensors for timing and fuel control have been replaced. The IAC has not been, but it makes no difference if it's unplugged or not. Throttle position sensor is new, MAF is new and clean, ECM is new, crank position sensor is new. Plugs and wires are new. Fuel pump is new as is the pressure regulator. DPFE sensor is new and differential pressure high is the only code the truck will throw.

Am I looking at an O2 sensor issue? No codes for it, but maybe? Should I just buy an IAC? Is it a stupid vacuum leak? Just throwing ideas at the wall here somewhat, also trying to build a paper trail for anyone else who might have this particular failure mode. Thanks!

cleaned the TB? on early 2.9s at least they was susceptible to sludge buildup there which caused a hesitation on cold starts iirc and sticking if the plate

 

[Nadams01]

Use a vacuum gauge when hot and compare it to vacuum at cold. look for a steady needle. Could have valves not fully seating or simply worn camshaft. If the vacuum is good at idle, chances are mechanically speaking the valve train is ok. If you can use a long vacuum line to monitor it while driving at partial load to see if it drops hard or is erratic, possible bad MAF if it does due to a incorrect calculated engine load causing the PCM to run rich or lean dependent on how off it is. You cant monitor calculated load on these OBD1s, its handled internally by the PCM but do clean the throttle body. With this high of millage Im wondering if the cat is clogged, could try removing it to see if the restriction goes away.

 

[wankel1308]

Put O2 sensors in and that fixed it.

 

[Nadams01]

Put O2 sensors in and that fixed it.

Typical OBD1, won't tell you anything you need to know. Nice fix!