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
Maybe the EGR valve the pintal inside maybe sticking and will cause the car to hesitate and choke.I am pretty sure ur model has one the easiest way to see if in fact that's the issue u can just disconnect the vacumm line going to it and it should clear up.If in the event there is no vac line remove the egr and clean it with carb cleaner and a wire brush and make sure the metal pintal that moves up and down moves FREELY.IF after cleaning its still not working buy a new egr valve.What happens is the daiphram inside is problem worn out and leaking air which in turn affects the car idle and running condition as a whole.That's my recomm to try first.I am not sure of the location of it on ur model check a manual unless u know what the part is or ask others in the first gen forums.
I have a similar problem, but not quite. I have a '97 Sunfire GT 2.4/5-speed, 89500 miles, since new. At 1500 - 2000 rpm or so when I get heavy into the gas in 4th or 5th gear I get a stumble or hesitation for a brief moment or two, then once the engine gets a little more rpm (2500) everything is fine. Also from a stop letting the clutch out and ringing it out 1st -whatever the car accelerates fine. If the car is in any gear above say 2500 rpm I don't have an issue. No codes are thrown. I am going to replace the fuel filter anyway, but I believe that since the car is fine under WOT at high rpms, I don't think it is a fuel capacity/pressure issue. I too thought of dirty/old injectors (but I tend to think 90000 is light mileage on injectors), I run Amoco or Mobil 93 since car was new, BTW. I would like to test the relevant sensors, MAP, TPS, EGR??
My question is how do you test them, I have a Helm (GM) service manual for the vehicle, and it tells of dianostics on each sensor using a Scan tool set to certain modes, etc, etc. Is there an easier way to test these? Can you use the OBDII scan tool you can borrow from Murray's to run the tests requiring a tool? Am I missing anything?
Thanks for your time.
You can use that scantool as long as it's not just a code reader. It must be an actual scanner which shows sensor data, such as voltage the ecm is seeing from the sensors.
well like stated above check the EGR..you said theres a rough idle..so are you misfiring?? if so check your injector resistance and the same goes for the coil packs, i don't remember if you have a distributer or coil packs. you could possibly have a plenum gasket leak(had that problem once, i used propane to detect it).
hm... does it run fine under heavy load? If O2 is working right and telling the truth it sounds like your running out of fuel. Fuel pressure can be in spec however fuel volume can be low and pressure will read ok. The only things that can cause low volume are a restriction, clogged filter (your already eliminated this), pinched or corroded line, clogged injector, clogged fuel pickup, and low pump RPM. The only way to check RPM is with a low amps probe, it is a more advanced diagnostics procedure not really for the do it your selfer. What are your fuel trims as well if they are high it you are defiantly running out of fuel. Remember 128 is base not 0
1979 Impala Sport Coupe Aeroback
Well I've changed some different sensors trying to eliminate some areas. All numbers on my autoxray look good. My blms are between 118 and 131, so it's actually pulling some fuel out. These numbers are alot better after I changed the crankshaft position sensor and fuel pressure regulator. I'm really beginning to think that 222,000 miles has put enough wear on the engine to do this, but not sure. I've been through the service manual and have done most checks. The only thing I have left would be the fuel pump, but I think it's fine, according to my blm numbers. It does have the speed density update to the PROM, which eliminates the MAF sensor. I would like to find someone who has the old original prom chip and get a copy of it or even buy it from them. GM does not offer this chip anymore. I have done PROM tuning on my truck and have burned many a chip for it. I feel that the speed density update didn't do this engine any good.. It's worth a try, there isn't much left to do.. here is a post about the PROM files I was going to put up and see what I get from it .. thanks for the reply!!
Hi all... I'm looking for some PROM bins for my 87 Z24 cavalier with a 2.8 liter
mpfi. I have the newest prom GM had for it, but I am looking for older
bins. GM no longer has any of these bins available. My bin has the speed
density update, which eliminates the MAF sensor (ATZA). I would like to use
the MAF sensor. Here is a list of broadcast codes my Z24 used throughout
the years. If anybody has one, please send it to me. The computer is a 1227730. Thanks to all!!
ADDK - original bin, need still
ADSF - need still
AMZL - need still
ASMR - I got this one off the GM ECM ftp site already
ATZA - latest bin, I have this in the ECM
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
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
