High revs and bad gas mileage now dying

I have a 91 4.0 4x4 auto with some strange symptoms. The gas mileage dropped dramatically and it will intermittently rev to 2-3k rpms for 2-3 seconds with any slight touch of the gas. Even if I barely tap it it will rev then settle back down. This morning after driving about 20 minutes it wanted to stall when I hit the gas. If I eased into it it would go but any real dip into the gas wanted to kill it. About 10 minutes and it went back to normal. On my way home it started out fine then same issue. I actually did die getting going from a stop and would crank but not fire. Got it to fire and limped it easing into the gas. then it cleared again and ran normal. TPS CPS and ECU seem like the most likely options from my research, but thought someone might be able to nail it down a little better. I would be randomly picking one to start with(probably tps because it is the easiest to get to!) Thanks.

Does seem like a TPS issue. I would probably start with that. You should be able to test it to confirm it's working.

Hook up a vacuum gauge and monitor it during the rev. My bet is you have a vacuum leak. Check the throttle body mounting bolts. Clamp off the vacuum line to the vacuum booster.

Take a look at the intake manifold gasket. Do the ether test.

TPS issue, or a vacuum leak to the MAP sensor that comes and goes, or a bad vacuum hose that gaps open when the engine moves. TPS is at the top of the list. Also be sure to check the battery / sensor grounds and clean them up if needed.

So it isn't the TPS. Replaced with no change. I lean more toward a sensor vs vacuum because of the heat related/intermittent nature of the problem. Would a major vacuum leak not be constant? The stumble will kick in at almost the same point in my commute every day (about 8-10 minutes run time). It will then eventually clear up as well(40 minutes in) although it has been lasting longer and coming back occasionally now. I can ease into the throttle and once off idle can feed it gas and it will drive, but if I dip into the throttle hard it will kill it. It idles fine all day. Hot restarts it cranks and cranks eventually starting with the throttle matted. Cold starts are always fine. I am stumped.

I will check vacuum again. I do get a hissing under the dash occasionally on braking. Booster? Also the vents revert to default immediately after I turn it off, so it obviously isn't holding vacuum that well, but why would it be ok one minute and bad the next, then back to ok if it was a bad leak?

Intake manifold exhaust gasket can change leak volume with heat/temp. Or a partly damaged tube or vacuum hose can change leak size with motion. There is a hidden vac hose on 91 and up, IIRC ??? going to the Map sensor that can go bad and have a variable size leak.

Sensitive gas pedal is usually is a TPS, MAP sensor vac hose leak (Throttle body) or an O2 sensor issue.

A stumble after warm up could be a bad O2 sensor, or lack of power to the heater in the O2 sensor. It is starting to sound like it may be an O2 sensor issue and an Air temp issue, but hard to blame just one issue for all the current symptoms.

Temperature related issues, some of yours can be a dirty or bad Intake Manifold Air Temperature sensor, severalof your symptoms match ones I had with a bad/dirty Air intake temp sensor.

Have you done the ignition key trick to check for codes?

Booster leak could be the noise, or an HVAC vacuum control tube is loose.

Also check the Fuel pressure regulator on the front of the fuel rail (91 still had one IIIRC) at the vacuum port to see if fuel is leaking into the vacuum port.

I would find the Vac leaks, fix them, then test the air temp sensor and the O2 sensor and wiring and power feed all first. The sensors can be tested with a volt/ohm multi-meter.

Ridered4583 said:

I will check vacuum again. I do get a hissing under the dash occasionally on braking. Booster? Also the vents revert to default immediately after I turn it off, so it obviously isn't holding vacuum that well, but why would it be ok one minute and bad the next, then back to ok if it was a bad leak?

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I have had hissing from brake boosters as they went bad, but they never did anything but spike the idle a wee-bit when I pressed or pumped the brake. So I doubt it is your problem.

I wouldn't think it's the booster but maybe the one way valve connecting to it. Easy and cheap to replace and also easy to test with a vacuum gauge.