Hesitates and backfires!

[greenz28]

I have a 94 z28 with h/c. It all of the sudden started hesitating really bad at low rpms and feels like it has no power. It hasn't thrown any codes. At idle it has a weird lope to it and doesn't sound right, but when I hit the throttle the rpms drop. It seems to smooth out a little at higher rpms. It is harder to start now too. Also I noticed that it is backfiring through the intake when I hit the throttle. I have perfect fuel pressure, new AC delco 02s, and it runs worse with the MAF unplugged. Thanks in advance for any advice.

[95formula383lt1]

mine is now doing the samething after pulling the headers off to do a compression test and attempting to hook up the pcv system. working on figuring it out today. my knock sensor looks broke from what i can tell skimming over it quickly just dont know if its the cause.

[95formula383lt1]

just replaced my knock sensor with a spare gave it a good beat down, and runs great now, u might want to check yours.

[greenz28]

just replaced my knock sensor with a spare gave it a good beat down, and runs great now, u might want to check yours.

What symptoms did you have exactly?

[95formula383lt1]

it would run ok in the drive way at idle for a min or 2 then when i tryed to drive it it would stumble hard in climbing rpms. it would hesitate bad trying to pull away from a stop sign or light and im i tryed to give it a rev to clear up it would pop through the intake. it would try to clear up a bit in high rpms, when i would come to a stop the rpms would fall lower then 400 rpms and barley idle had to run a red light cuz i lost the brakes cuz of vacuum. there was a few more symtoms i dont remember right now but my knock sensor plasic tip was all loose an the seal was all ratty looking, my buddy jus had the same problem but his sensor looked like new we fixed his now his seems to be better to the only thing is his is a 96 and it was throwing the code.

[greenz28]

Mine idles weird, but doesn't idle low. The only time it drops rpms is when I hit the gas. It has these same symptoms from the time I start the car to the time I cut it off. Did your friend have the same symptoms you had? I'm starting to think it is my opti. I changed the thermostat the day before it started doing this, but I didn't THINK I got any water on it.

[95formula383lt1]

i dont know what all his symtoms were they were simular to mine as far as i know. its hard to change ur thermostat and not get any water on the opti. It very well could be the opti, mine would only run though till it got hot, then would only start after a few hours of it cooling off when my opti went out but mine is vented opti i dont know much bout the early optis. good luck though.

[greenz28]

I swapped my opti, so it is vented as well.

[lt1truck]

check for vacuum leaks, maybe something you forgot??

[greenz28]

I have searched and searched for vac. leaks and burned wires.

[meissen]

Do either of you guys have an EGR valve still? My 3.4L v6 kinda did the same thing when my EGR valve started sticking. I can't remember if it threw codes or not - it obviously does when it's entirely yanked out but a mildly easy test would be to unplug the EGR valve and see how it runs.

[greenz28]

Sorry, I forgot to mention that my egr has been deleted and tuned out. Is there any way to test the opti before I buy one? Does this even sound like an opti symptom? I didn't make very clear how bad the car is. I wasn't sure it would make it home. It started out as just a bog when I pressed the gas quick, but didn't do it if I eased into it. I got on the car one good time and when I let off it had the symptoms I described. It wouldn't even pull up a small hill until I floored it. When I hit wot for a sec. it seemed to clear up until I let off, but was still weak. I went out the next day to try to figure out what happened. I started the car and it had that weird lopey idle and when I reved it, it sputtered and backfired through the intake. I know it was coming from the intake because I saw the elbow bulging out.

Sorry for the long post.

[lt1truck]

Hmmm.. an easy thing to do would be to take your ignition control module off and have it tested, napa can usually do that, but mine had somewhat of the same symptoms whein it took a dump. Its just a free and easy thing to check

[greenz28]

Thanks, I will give that a try today when I get home.

[meissen]

I have heard a few stories of people taking it into high RPMs and the rotor blowing up - might want to check the optispark after all.

[greenz28]

Hmmm.. an easy thing to do would be to take your ignition control module off and have it tested, napa can usually do that, but mine had somewhat of the same symptoms whein it took a dump. Its just a free and easy thing to check

http://ltxtech.com/forums/showthread.php?t=5606&highlight=backfiring+intake

This was your icm? This sounds alot like my symptoms. I just always thought that it wouldn't start with a bad icm. I have a video of the car acting up, but I can't get the sound to work. As soon as I hit the throttle it pops and sometimes cuts off, but about 3000 rpms it seems to clear up.

Also what exactly does the icm do? Does it control timing in any way?

[lt1truck]

Yeah thats my thread i never thought that the icm would have anything to do with that either, but i broke down and spent the $120 on a new one and sure enough that was the culprit. I bought one from oreillys before i knew napa tested them

[greenz28]

That would be a lot easier and cheaper to fix than the opti. I will let you know if it fixes the problem. Thanks!

[lt1truck]

Oh i forgot, i dont know if you already know this but if you could space the module and coil away from the head it will keep it cooler and problem free for longer, i used some old intake studs to space mine out a bit!

[1FASTSS]

Check for spark, then check for fuel. Its one of the 2.

Coil
plugs/wires
opti
Fuel pressure good?

[greenz28]

Well, I just finished changing the icm and coil as well. It is still running the same. Any other options before I have to dig the opti out?

[lt1truck]

did you have the icm tested first? i swear this sounds just like a big intake or vacuum leak, i think this because i adjusted my rockers with the engine running and forgot to put the brake booster line on when i went to start it and it had a tough time starting and when it did it would idle all "bumpy" and backfire out of the intake real bad. you should go over every line including pcv, intake boot, brake booster line, pull off the fpr vac line and smell for fuel, maybe the diaphram is ruptured, thats all the stuff i would check before pulling the opti. did you put new plug wires on recently? maybe a crossed one or burnt by the header, it would suck to slap a new opti on and still chase this problem, im not saying its not the opti but throw money and parts at a problem rarely solves it. Hope this helps

[greenz28]

The plug wires are not crossed, because it was running fine before. I have searched everywhere time and time again for vac. leaks and burned wires. I let the car idle for a while today and noticed that it is running rich. Also after idling for about 5-10 minutes the car died, and was a little hard to start back. I am gonna swap the tps and ks tomorrow with others I have laying around just to rule that out. I checked the fp the other day and it stayed above 40 psi. I really wanna get this problem solved, because I just finished installing my 12 bolt and MT ETs today and wanna try them out.

[lt1truck]

jeez this is leading closer and closer to the opti, i guess if you have ruled everything else out you could pull the opti and yank the cap off to see it it has any signs of stray spark in there, theres a way to tell you might be able to google it and find out but yeah i guess thats the next step.

[greenz28]

Just ordered a new delphi opti for almost $300. :(

[greenz28]

Just wanted to give everyone an update on my backfiring and hesitating problem. I just got the new opti put in and it runs great now. Thanks everyone.

[lt1truck]

good!! now dont get it wet haha!!

[greenz28]

I know! I packed rags around the new while I filled the coolant up.