Alrighty guys, you've never failed me before. This one is for the really smart guys out there(*cough*vader*cough*) I have a 91z28 5spd 305. Yesterday, about 5 miles after filling up with regular Texaco gas, the car started misfiring REALLY badly in the midrange. The car will idle fine, and doesn't misfire at WOT, but anywhere between idle and WOT it misfires to the point where it doesn't accelerate. I thought it could have been bad gas, so i went and got some octan boost and put it in, it didn't help. To get anywhere, you HAVE to drive at WOT.:rockon: The computer is throwing no codes. Today i went through and replaced all the spark plugs and checked for loose hoses and the such. There is one catch, every couple minutes, it will drive fine, like nothing is wrong. Then just like somebody flips a switch it goes back to misfiring. I took it over to a mechanic friend and he was baffled. He thought maybe it could be a wire grounding somewhere or maybe the MAP sensor. FYI I replaced the o2 sensor two weeks ago, it is running an aftermarket cam, and runs fine in limp home mode. Any ideas or suggestions would be much appreciated. You guys are a very valuble to all of us!

 

I'm not a "smart guy" like *caugh* Vader *caugh* or anything... but I'd replace the ignition module in the distributor and watch your problem go away.

 

if the ignition module was bad wo uld it allow the car to even start?.. i say this because a while back my car died at the track and wouldnt start back up. after towing it back to my house i replaced the ignition module and everythings been aok since....

 

Yup I agree with the ignition module. And it will start with a broken 1.

 

It most likely is the iginition module. I've had to kill on me one the car completely died and wouldn't even fire. The other it missed really bad at high rpms.

 

Sounds like when my module went, just like someone flicked a switch. My idle sure wasn't 'fine' though. It took me about 5 minuets worth of diagnosing with my motors manual to figure it out. If you find it is your module, be extra careful of the internal two-prong connector when changing it; mine was so fragile it fell apart in my hands.

 

Just replaced the ignition module, and that wasn't it. It is still doing the same thing. Any more ideas?

 

Did u do it correctly and put that grease stuff (don't recall the name) on it right??

 

pretty sure i put it in right, and yes i used the dielectic grease to promot heat transfer to the distibutor.

 

Maybe the octane boost would've cleared this up, but you might try putting in some "heat" additive in case it's got any H2O in it. Might also try getting some fuel system cleaner additive in case the gas was just put in their tanks--it can stir up the crap and sediment off the bottom. Might also check plugs and wires, although I'd think if it was something like that it would get worse at high rpms. I'd also check your cat. converter. Mine made the car act like yours is when it went out...before it actually blew the converter guts through they get clogged and the engine cant breathe.

 

Can somebody check they're Hay's manual and see what possible causes for misfiring in the midrange are? I can't seem to find mine at the moment. Thanks!

 

o yea, and i already tried adding octane boost, it didn't help. Havn't tried any of that other stuff though, the tank is down to one quarter, maybe i'll go fill up with some premium.

 

I have heard of the distributor pickup coil playing similar odd tricks when the paper film surrounding the coils gets brittle and takes a crap.

 

this happened to me in my old 85' TPI trans am. i had just filled up my tank, and while leaving the station, the car fell on its face at about 1500 - 2500 rpms. i had to step on and off the throttle to get it to eventually accelerate. turns out the problem was a bad fuel pump. hopefully this isnt your problem, because they are an absolute PAIN in the *** to replace. good luck!

 

check fuel pressure. you'll probably find your problem there.

 

You should check both tps and maf sensor.

 

might also check the fuel filter.. if you ran real low of gas you may have picked up some massive junk fromt he bottom of the tank. i had a problem a while back with my car running like crap and i changed out the fuel filter, been alright since.

 

does anybody know if/how i can test my MAP sensor?

 

A bad EGR valve might also do this. Also like other people suggested check or replace the TPS sensor also.

 

just a note octane boost dosnt do anything when its put in after you fill up, you should put it in before hand

 

Alrighty then, here's another piece to the puzzle. I went out and spent the $40 on a fuel pressure tester today and found the pressure to be about 25 at idle with and without the vacuum hose on. This leads me to believe there is something wrong with the fuel system. Like previously said, why would the car be fine sometimes, in between other times, and absolutly horrible other times. I'm so confused. I think i'm gonna take it to a shop for the first time in over two years unless you guys have any more ideas.

 

I agree with a post given above abot the pick up coil....My 1987 GTA did exactly the same thing.....I was baffled for a month replacing everything from the regular coil, plug wires plugs ignition module.....Hell even the diacom showed nothing wrong... After reading the archives I found a thre=ad about the pick up coil...I tested mine and it was bad....Replaced it an NO PROBLEMS since then....

BTW...Mine would Be fine foor DAYS and then out of no where act up....Id change something and it was fine...then 215 minutes into a trip it would act up...Very very frustrating...And worth a try...

 

when i had my problem it was suggested i change the ignition module and if that didnt work that i changet he pickup. so try the pickup coil....

mike

 

im still casting my vote for the fuel pump

 

I just had a similar problem with my car. It would go fine until about 3000rpm then it would not go. My problem was one of my cats came apart form the inside and was clogging the exhaust. My second guess would be the other thing that went bad on my car a few weeks ago.....the fuel pump. Its really fun changing that too

 

It could be the fuel pump but Im suspect of that because he said it runs good at WOT...If the fuel pump was getting lazy or bad altogether WOT would be the worst as the car would be running horribly lean and be preigniting ect ect.... But it is a valid possibility...When I noticed my fuel pump was going bad I suffered greatly at WOT...

 

the shop wants 475 to replace the fuel pump, is that a good deal?

 

Im not sure what the going rate is on a pump install... If I were you and I was having one replaced I go Ahead and get a HIGH flow walbro pump...They plug right in and bolt up just like stock but will give you more fuel when you are ready for it...That way if you add mods your fuel pump can support it...If not you will never know its there...Plus the price difference is negligable...

To do it yourself is a PITA...I did mine in roughly 6 hours...Split up because I broke the filler neck taking it out of the car.....Plus I have a lift....I would not want to try that job with tyhe car on some jackstands.......

 

still bet on the fuel pump. the reason why it would be fine one time then not another is because the fuel pump is getting hot and slowing down. but one thing you can do is pinch off the return line and see if the pressure goes up. if so, then possible regulator. but i still bet on the fuel pump. did it on a friends car. replaced every sensor and still ran crappy. i put my OTC scanner on it and it was running lean. the o2 was reading lean. replaced pump and problem solved. but try the pinch test first.

 

Hook the fuel pressure regulator up and put it on the windsheild so you can see it when you are driving. Drive the car. Ithink stock fuel pressure should be between 37 - 42 psi. If the pressure is dropping off badly or jumping around erratically, then a bad fuel pump or regulator is most likely your problem. If it seems to be fine, then your problem will be most likely an ignition problem. Make sure all of the wires inside of the distributor are making good connections. Look closely at your distributer cap for burns or cracks. I had a simmilar problem that was caused by a screw that holds the coil being tightened to far and it was comming through the inside of the cap. If all of that fails, I would look for vacuum leaks. Good luck!

 

Don't know if you've solved your problem yet, but I had the same ting happen on my 2.8 MPFI. I took the fuel line loose before the filter, put it in a bucket, and turned the key on. The flow was fine. So I went through and replaced the ignition control module, cap and rotor. Still had the problem. Finally took it to a GM dealer, who couldn't find the problem on the first try. After I picked it up, it did it again on the way I home. I replaced the fuel filter just for kicks and took it back the next day. They said my cap was just a little loose, but I remembered tightening it down right. So wheter there was a problem with the cap, or if my fuel filter was just clogged, it never happened again.

 

Just for the record, it was the fuel pump

 

Did you do it yourself or pay someone to do it.... If so, what did it cost? Previous owner said he replaced the one in my car about a year ago, so hopefully it has many happy days aheads of it, but that is one problem I dread. My brother replaced one in one of his trucks... it's easier than a Camaro, but he takes the bed off the truck to get to it!

 

it cost me 475 and it took the shop about half a day to do it. They replaced my 305 fuel pump with a 350 fuel pump just in case of future upgrades.