Hesitation Issue, Loss of Power! NO CODES PRESENT!! Wtf!?

**LoneGunman** · 05-06-2013, 07:30 PM

I need some help here fellas. I've got a '99 Z28 M6 with 137,000 miles on it, and I am having a problem with the way it runs. It absolutely HATES 1500 rpm's! It sputters, and surges kinda like the way bad plugs or bad gas would act. Cruising at 60mph in 6th gear is a total PITA! If there is a slight incline I have to downshift to 5th, otherwise the surging gets worse! When you get on it, it'll go but I can feel the motor just "laying down". It doesn't have the power or the smooth acceleration that it should have. I have changed the plugs (NGK TR55) and the wires, changed the fuel filter, ran a few tanks of 93 octane and two bottles of Lucas through it, cleaned the MAF and the TB and it still does it! My buddy hooked his 4 thousand dollar Snap-On Scanner up to it today, and according to that everything is functioning properly. The only thing he took notice to was that the bank 1 rear O2 sensor readings were a little erratic. I was thinking it could be blocked cats! It has a LM1 exhaust, and is not as near as loud as it should be! I am at a loss and need some expert advice, please help!

**JayTA98** · 05-07-2013, 04:45 AM

well im no expert but i think if you had blocked cats you will have issues even when the car is at idle. Is this friend of yours someone that you trust and really knows what he is doing? No disrespect intended. Did he check the fuel pressure at the fuel rails? You didn't mentioned if he checked that. The code could indicate you are having an issue with that bank and it can even be a cat.

Have you rechecked the spark plugs? Did you gap the plugs before you put them in? I had put new plugs and when i have a surge after that I recheck them and found one that was cracked casing it to arc. check the wires and make sure the are seated right. the side close to the coil needs to click in twice.

**Knight** · 05-07-2013, 04:55 AM

Agree with Jay. Check the fuel pressure at the rail. Also, if you're running LT's, did you install O2's with the longer extensions or did you splice them in?

**JayTA98** · 05-07-2013, 05:01 AM

Originally Posted by Knight

Agree with Jay. Check the fuel pressure at the rail. Also, if you're running LT's, did you install O2's with the longer extensions or did you splice them in?

good question Knight. I had a friend had extensions and he had multiple issues. He ended buying the longer cable O2 sensors and problem solved. I just wrapped the excess cable around the engine mount base so if I do headers in the future I don't have to buy O2's again.

**LoneGunman** · 05-07-2013, 03:42 PM

I don't have LT's, and yes I trust the guy who checked it out. We did it in his driveway. It did this even before I changed the plugs. When I pulled the plugs out, there were 2 different kinds of plugs on the pass side, so I thought that maybe that was the problem...but...nope

I replaced them with NGK TR55 gapped at .55. I have not checked the pressure at the fuel rail though. What is the pressure supposed to be at the rail?

**Knight** · 05-07-2013, 05:55 PM

You should see 58-60 psi.

**LoneGunman** · 05-07-2013, 06:11 PM

Ok, thanks. I'll check the pressure and post the results.

**theorangeguy** · 05-08-2013, 05:36 AM

sounds pretty simliar to what I just dealt with...I would check the o2's and clean the MAF if the fuel pressure is solid. Does it idle and sound fine when you are sitting still "like its running on all 8"? Also, does your oil pressure bounce around a lot under load? I had a 99 Z28 that had cats and I had one get clogged and it crippled the car...if yours has cats, can you look at it from under the car and see if its discolored because of heat? If its clogged, a lot of times they'll start to change colors because of heat build up...

**b.lee** · 05-08-2013, 09:13 AM

I had bad cats on my 03 z71 5.3L ....

It just got slower, and slower, and more hesitation and what seemed like a misfire but it was fine at idle.
What I mean by slower and slower is you had to give it less and less throttle for it to stop bucking and misfiring.

Banged on the cats with a hammer... got a little further down the road and it started misfiring and bucking even at idle.

So they might be partially plugged... unhook the exhaust right before the cats if possible and run it... see what happens.

**LoneGunman** · 05-08-2013, 05:51 PM

I checked the fuel pressure today. It was at 60psi, never dropping below 58. So that is fine. It idles, revs and runs fine. It's just a "falling on it's face" feeling when you get on the gas. Trying to hold it at a steady RPM makes it worse. It just feels like and old truck with bad timing. Idfk, I ordered LT's and ORY last night, so maybe after I install them and have it tuned that will help.

**theorangeguy** · 05-13-2013, 05:53 AM

Originally Posted by LoneGunman

I checked the fuel pressure today. It was at 60psi, never dropping below 58. So that is fine. It idles, revs and runs fine. It's just a "falling on it's face" feeling when you get on the gas. Trying to hold it at a steady RPM makes it worse. It just feels like and old truck with bad timing. Idfk, I ordered LT's and ORY last night, so maybe after I install them and have it tuned that will help.

It really sounds like a clogged cat to me...I lived around the Atlanta area for about 5 years and had to replace 3 cats on a Z28 while I lived there because of emissions and everytime one would get clogged it would do exactly what you are talking about.

**oldguy** · 05-13-2013, 07:25 AM

I had the sme problem with a 5.3 in a silverado. It kept getting worse until it would barely pull itself up my driveway. Took out the upstream o2 sensors and it sounded like shit but ran alot better. Gutted the cats and it is still running like a champ.

**theorangeguy** · 05-13-2013, 07:55 AM

Originally Posted by oldguy

I had the sme problem with a 5.3 in a silverado. It kept getting worse until it would barely pull itself up my driveway. Took out the upstream o2 sensors and it sounded like shit but ran alot better. Gutted the cats and it is still running like a champ.

I am kind of surprised its not throwing a code yet...mine always did when they got really clogged up.

**oldguy** · 05-13-2013, 07:58 AM

All I ever had was a flashing CEL if you stayed on it once it hit the flat spot. If you back off as soon as it dies out I am not sure it would give a code.

**LoneGunman** · 05-13-2013, 08:05 PM

I have no clue why it's not throwing a code. I guess it's because it must not be bad enough to the point to throw one. I just got my headers and ory yesterday. I plan on tackling that this weekend, and we'll see if that helps. Right now I'm wrapped up with window problems, so the car's been in the garage for the past couple of days

**PAFirefighter11** · 05-14-2013, 05:28 AM

Not to scare you but this is how my broken valve spring issues started.... Just kept getting worse and worse and then it was almost impossible to drive.

**LoneGunman** · 05-16-2013, 10:25 PM

Originally Posted by PAFirefighter11

Not to scare you but this is how my broken valve spring issues started.... Just kept getting worse and worse and then it was almost impossible to drive.

Uh Oh!! That doesn't sound good. I really hope it's just a bad cat