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

Did u check the EGR valve yet? I posted a reply for u in maintenance I think that mayb ur problem.If after doing what I recomm that does not work just let us know and we will go from there.

I am going to head up the the muffler shop this morning, and get my exhaust all checked and touched up. Then I'm going to pull the EGR valve out and make sure everything is clean and working. I'll post what I find and see where I can go for there. Hopefully that is all I have to do, hehe, thanks for the reply!!

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!!

Another update.. I checked compression and all cylinders are within 10 psi. Ok, now for the bad/good part..hehehe.. I have a dead cylinder. I have great spark, but no fuel, hmmmm. I've used different injectors, so my guess would be the wiring to the injector since the other two injectors on that batch are firing. So all I have to do is pull the wiring to that injector and check it out and/ or replace it. The wires are all very hard and brittle, so it wouldn't surprise me that there is a short in it. I'll keep you all posted, Thanks for the ideas so far!!

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.

Good to here u half the problem resovled.I will think on the idle issue and let u know if I come up with a idea.Hope the O2 sensor helps but,not sure that will resolve it.

You're right, the O2 sensor won't do a thing for me, so I'll scrap that idea., I just got my service manuals in the mail, so I will be checking them for anything. At the moment, I feel that I'm not getting enough fuel right off idle. I'm going to check my ECM today and see if it has the speed density update. Who knows what this vehicle had done to it before I got it. Anyone with ideas, just shout them out!!

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

that's great news , i may need those charts i have been having some ruff running issues to but i ave not look in to it att all yet but this tread will most likely help me

Good stuff man. I just read all of this to try and help and........nevermind you did it yourself. Good work. I do believe that the speed densisty software upgrade took care of this problem when they disposed of the mass air flow and used the map instead. I know my 87 had the same problem, i put a chip in it and it solved the issue at hand.



My cardomain site
doublez@eastlink.ca

What did you use to re-program your PROM? It sounds like I'm having the same problems. Ever sice I put in a bigger cam and rebuilt the motor it sjust hasn't been the same. When you get an opportunity I would like to have you E-mail me the tables hat you used to re-do your PROM.