Okay, I did extensive searching on the forum about this, and while I got a couple of ideas, no one's problem appeared to be exactly like mine. So, to clear things up in my mind, I decided to post exactly what is going on with my car.



Okay, got the car a few months ago. It's a 91 Z-28 5.7L, and it always had a little hesitation problem and at low RPMs, it would occasionally buck and jerk. When it did this, if I floored the accelerator, the bucking would clear up, and the RPMs would rise quickly and smoothly. Never any SES light.

Fast forward to present, changed oil/filter, trans filter, air filters, and fuel filter. Car will start up fine as long as it's been sitting for at least a few hours. After a bit of driving, the SES light will kick on and I have to either keep pumping the accelerator, or keep the RPMs really high (don't worry, I don't run it into the red). or the engine will sputter and die. If it dies and I try and start it, without any pedal, it will immeditally die, almost as it I turned the key off. Checked codes and got the usual 12 and code 33.

Any ideas are greatly appreciated. Thanks

 

sounds like a bad map sensor, check the hose for cracks, and check the electrical hookup, later jimmy

 

Code 33 = MAP sensor or something directly related to it as mentioned above.

 

Thank you so much. Checked out the MAP sensor and all attaching h/w (connector and hose). Ended up replacing the sensor itself, and the problem seems to be gone!!! No hesitation, no dying, and no 'bucking'. Although, at idle speeds sometimes, it does seem to fluctuate the idle between 5G and 10G, not quite sure what to make of that.

Anyway, thanks again!

Mark

 

Okay, so the map sensor is new, and everything around it seems to be gtg, and the car definitly runs better, but occasionally, the idle will start fluctuating and surging, and car will die. Next start, will be fine. THrowing same code. any ideas.

 

A trouble code does not necessarily mean "the so-and-so is bad, replace it".

The ECM has no way of looking out at most of the parts attached to it, and telling whether they're bad or not. No way to measure that. A few things, it can at least to some extent, like injectors and solenoids; but alot, especially sensors, it can't.

What a code like that means is, the ECM is seeing a set of inputs from the various sensors, that cannot possibly exist. A good example would be: low RPM, high throttle position (wide open), low MAP (high vacuum). This set of inputs CANNOT exist if everything is working right, because that basically means that the RPM is low, the driver has the pedal floored (which should result in VERY LOW vacuum), but instead the vacuum is high, as if the engine is idling. The ECM doesn't know which input is wrong, but it can tell that something CLEARLY is FUBAR.

I'd suggest checking your TPS voltage; maybe wiggle the throttle a bit and see if it does anything weird, maybe try checking it WHILE IT'S IN THE ACT OF SCREWING UP, and see if it's wacked. Should be around .55V if the TPS is working right. I'm betting you'll see it intermittently shoot WAY higher than that, and maybe when it's screwing up, it just goes to 5V or something, and stays there. If the TPS voltage just goes high like that and stays, the ECM won't have any other way to know that the driver DOESN'T have the thing floored (it doesn't have little eyeballs looking at the gas pedal), and instead of recognizing it as a TPS error, may think it's a MAP sensor error.

Or, just change out the TPS, since it's cheap and easy. Just throwing parts at something isn't usually the best thing to do; but sometimes, a well-reasoned parts swap, not just a scattergun "maybe it's this, maybe it's that", might be quicker, easier, and cheaper than trying to actually troubleshoot it. Especially with an intermittent like that.

 

I had a similar problem, idle surge from 500 to 1000 rpms while at a stoplight or stopped. It went through nj emissions inspection, and results told me it was running rich. Hydrocarbons and CO were through the roof. So I had previously seen the code that said O2 sensor lean. The code was not present in the system at all times, i only saw it once. Knowing that the car was running rich, I replaced the o2 sensor since the computer thought the car was running too lean, and adding fuel trying to compensate.I replaced the sensor since i had a new one in my garage that i bought for my pickup and never installed..and it was the same sensor. Now this car runs much better and passed emissions with flying colors. I'm not saying your problem will be o2 sensor related, but it is a possiblity. It sounds to me like your car is running rich, so you may want to test components that relate to air fuel ratio-tps sensor, map sensor, 02 sensor, injectors, vacuum leaks, fuel pressure, etc

 

hi have a similar problem just start sunday, it hesitates, looses power, if i floored the petal there no response, and also my engine is viberating badly sometimes after not to long putting some gas in my car, my check engine comes on sometimes and then goes off, ran the code and came back o2 sensor about 3 weaks ago, change that yesterday along with the wires, plugs roto & cap and air fiter

 

where is the map sensor located