My old 2.8 MFI in my 87 Z24 cavalier has a hesitation when coming off an idle. When I press the gas it bogs and the O2 sensor goes to zero for a second, then it catches back up. It only does it from an idle. If you already had some rpms, it seems to work just fine. There are no trouble codes. I used an Autoxray scanner and fuel pressure gauge to help me diagnose this. I have done some tests and they all come up good. Fuel pressure is good before startup and at idle. The pressure does come up as I accelerate due to the vacuum controlled regulator. So I don't think it is a fuel pressure problem. I also changed out the fuel filter for kicks. All the sensors look good looking at the readings from the Autoxray. Good volts throughout the range for the TPS and MAP. The O2 sensor is cross counting correctly until I touch the gas off idle. I changed the plugs and wires to eliminate that area. It does seem to have a little bit of a rough idle. The motor does have over 200,000 miles on it. Maybe injectors getting bad. I have had a similar problem with a 4.3 blazer when the cat was plugged right up. It makes a heck of a bog sound from the air box when I quickly touch the gas, as the blazer did, hmmm. I'm somewhat stumped why it is sooo lean when I hit the gas off idle. Any ideas of what I could check next. Do I need some different tools to help. I'm willing to get what I need. I do alot of this type of thing, so it wouldn't be a waste to get them. I don't know the history of this vehicle. The car is a manual, if that helps. Thanks!

Andy
82 Z28 Camaro built 355
87 Z24 Cavalier 2.8 stock
90 Lumina 3.1 stock
93 Fullsize Blazer custom built Vortec Holley TBI
http://www.bds-suspension.com/pride_joy151.html :banghead:

I have a 89 z24 with almost 200,000 miles on it with the 5 speed and i have the same problem. I have just replaced the fuel pressure regulater. Do to the fact it was letting gas in to the vacume lines. when i got it back together i had this problem. Well i was playing with it the other night and i didnt have the car running 5 min when my brother noticed my exhuast was glowing. It was not the cat glowing it was the muffler and tail pipe were cherry red. I have cut the exhaust off the car to see if it has made it run right becuase the thought it was the cat too. But i got sick in the middle of the process and have not started the car. I hope this will fix your problem.

I have a 89 z24 with almost 200,000 miles on it with the 5 speed and i have the same problem. I have just replaced the fuel pressure regulater. Do to the fact it was letting gas in to the vacume lines. when i got it back together i had this problem. Well i was playing with it the other night and i didnt have the car running 5 min when my brother noticed my exhuast was glowing. It was not the cat glowing it was the muffler and tail pipe were cherry red. I have cut the exhaust off the car to see if it has made it run right becuase the thought it was the cat too. But i got sick in the middle of the process and have not started the car. I hope this will fix your problem.

Your problem is unburnt gas, rev it and it will throw black smoke out the pipe. Plugs and leads!

MrFun71, I would say that your injectors are clogged. i would say fuel starvation although it could be too much back pressure, just put your hand over the tailpipe and get someone to rev it and see if you feel the gasses if not it's blocked if so fuel starvation, just trry injector cleaner as a start in 91 + Octane gas and if that helps but doesn't cure it then I'd get them cleaned at a shop. I would suggest fuel filter and getting the pump tested at AutoZone but you don't think the problem is back there!

Thanks for the replies. I was gonna take off the upper plenum to check things out, so that would be a perfect time to check my injectors and regulator. I think I'll wait on the pump since that requires dropping the tank, not fun, hehe. I have to get a few things for the car anyway, so I'll pickup another set of low mileage injectors at the same time. I already ran injector cleaner through them twice, and nothing changed. Thanks again!! One more thing to add.. it bogs worse when it's warm then when it's cold and just started. If that changes anything, just reply.

82 Z28 Camaro built 355
87 Z24 Cavalier 2.8 stock
90 Lumina 3.1 stock
93 Fullsize Blazer custom built Vortec Holley TBI
http://www.bds-suspension.com/pride_joy151.html

Update!! Well I just got done changing the injectors. Nothing, no change, maybe even worse, hehe, can't tell. It really doesn't have the power it should. My exhaust is sounding like it's getting bad (leaky, noisy), so I'm gonna run her up and have a new exhaust put on. If it doesn't fix any of the problem, at least I'll have a quiet, powerless car, hehe. Any other ideas? Just shout them out..

Update!! Well I just got done changing the injectors. Nothing, no change, maybe even worse, hehe, can't tell. It really doesn't have the power it should. My exhaust is sounding like it's getting bad (leaky, noisy), so I'm gonna run her up and have a new exhaust put on. If it doesn't fix any of the problem, at least I'll have a quiet, powerless car, hehe. Any other ideas? Just shout them out..

even though you don't think it's the problem fuel filter!

My old 2.8 MFI in my 87 Z24 cavalier has a hesitation when coming off an idle. When I press the gas it bogs and the O2 sensor goes to zero for a second, then it catches back up. It only does it from an idle. If you already had some rpms, it seems to work just fine. There are no trouble codes. I used an Autoxray scanner and fuel pressure gauge to help me diagnose this. I have done some tests and they all come up good. Fuel pressure is good before startup and at idle. The pressure does come up as I accelerate due to the vacuum controlled regulator. So I don't think it is a fuel pressure problem. I also changed out the fuel filter for kicks. All the sensors look good looking at the readings from the Autoxray. Good volts throughout the range for the TPS and MAP. The O2 sensor is cross counting correctly until I touch the gas off idle. I changed the plugs and wires to eliminate that area. It does seem to have a little bit of a rough idle. The motor does have over 200,000 miles on it. Maybe injectors getting bad. I have had a similar problem with a 4.3 blazer when the cat was plugged right up. It makes a heck of a bog sound from the air box when I quickly touch the gas, as the blazer did, hmmm. I'm somewhat stumped why it is sooo lean when I hit the gas off idle. Any ideas of what I could check next. Do I need some different tools to help. I'm willing to get what I need. I do alot of this type of thing, so it wouldn't be a waste to get them. I don't know the history of this vehicle. The car is a manual, if that helps. Thanks!

Andy
82 Z28 Camaro built 355
87 Z24 Cavalier 2.8 stock
90 Lumina 3.1 stock
93 Fullsize Blazer custom built Vortec Holley TBI
http://www.bds-suspension.com/pride_joy151.html :banghead:


The fuel filter has been changed, and fuel pressure is looking good.

The fuel filter has been changed, and fuel pressure is looking good.

prob the exhaust then unless the cam and lifter are worn and you are getting very little lift and therefore very little air and fuel induction! 200,000 miles after all!

Update... Well my cat was in pieces and muffler had parts of the cat in it, so the new exhaust couldn't wait. I pulled the EGR valve, and checked it out. It moves free, the ports are actually really clean, I was surprised! One problem... nothing has changed! It almost runs a little worse, hehe. Starts right up every time and idles a little rough. I started pulling one sparkplug wire at a time to check for a miss. Couldn't find one. When I bring the rpms up to say about 2000, everything smooths right out, hmmm. I know from my previous Z24, this car has half the power my other one did, so I know there is something up with it. I had an ignition module and coil packs laying around, so I changed that out just to see if there was an intermitent problem. Nope, nothing changed. Exhaust smells really rich at idle and on my autoxray showing my blms up to 153 (adding lots of fuel) at idle, but then comes back down to normal range of 128 when bringing the rpms up. The exhaust crackles a little when I juice it and let off the gas also. I'm starting to think there might be internal problems with the heads. Burnt valves or maybe a broken spring, not sure though. I'm going to run down to a local shop and see what they say about it. Then I'm going to do a compression test on all cylinders. This should tell me if my valves are working correctly. It does have 222, 000 miles on this engine, so it wouldn't surprise me if there was a problem there. I'll check back later. ANY IDEAS? Thanks!!

Yet another update.. well I pulled the plenum again and replaced the injector harness. Good news, I have all six cylinders running again, woohoo.. The power is back!! Idles alot better now too. One problem, I still have the hesitation (bog) off idle. It really doesn't do it when the engine is first started and cold, but when it warms up a little bit, it starts bogging off idle. Basically when the ECM enters closed loop and starts using sensors to adjust engine parameters is when it happens. I am not pulling any codes from the ECM. The scanner numbers are within spec too. The only thing I can think of is that a sensor is lagging or not responding fast enough. Does anybody have any ideas?? Starting to get a little frustrating. I will probably go ahead and change the O2 sensor tommorrow and see what it does.. I might even try a different ECM, who knows? I'll keep everyone posted.

Well all, I'm back with good news!! This is kinda hard to believe, but to fix my problem, I had to reprogram my PROM. I talked with Howell Engine Developement. They were the guys that help me get started on the tuning for my truck engine. He gave me some ideas on where to look, and low and behold there were a few problems. There were a couple of tables that had a few problems. The Spark Timing table was off in the low rpm and high kpa area and two Acceleration Enrichment tables needed some smoothing out in the same area.. Seems weird, but the engine runs great now, no hesitation. Thank you Troy from Howell!! If anybody wants to actually see the tables or anything, let me know and I can send them to you, or I can try to post them up. Just amazing how such small changes have such a great effect on performance. Maybe I will be tuning this beast a little more, hehe. Thanks to all who have help in this thread!!


82 Z28 Camaro built 355
87 Z24 Cavalier 2.8 stock
90 Lumina 3.1 stock
93 Fullsize Blazer custom built Vortec Holley TBI
http://www.bds-suspension.com/pride_joy151.html