Missfiring at 5000 rpm

**redbird555** · 11-10-2011, 01:14 PM

I had this in another section but I didnt get too many hits so maybe it will be better here.

Anyway I've got some kind of gremlin going on here... Last week I drove back up to college (200 mile drive) and I got into the throttle a couple times and the car shifted fine at 6500 no issue. Fast forward 6 days later, now I dont drive the car much up at school and when I do I rarely get on it.

Last night I jumped on it for a couple secs and it ran great I didnt look at the tach to see where it shifted but it hauled ass and shifted fine, today I take the car out and decided to do a wot pull, i had the shifter in 1 (not the brightest idea i know) and hit it from 20 to eliminate the downshift lag well when it hit about 5k the car started to break up pretty bad and had no power.

At first I thought the tach was just slow and I was hitting the limiter so I tried it again but left it in D the car downshifted fine but at 5k it started to break up very badly again and I just let off at that point the tach instantly dropped to 0 but everything else was still running fine with no weird noises. It stayed that way until I got a block from my appt and then the tach suddenly came back up and worked ok. I got back, popped the hood, and theres no weird noises coming from the engine, it drives fine at low rpm as best as I can tell, no codes either. I also revved the car to 6k in the driveway and it didnt breakup at all.

When I saw the tach drop to 0 I immediately thought of a bad cam sensor, but I've had one go bad before and in that case my tach never started working again and the car had a hard time starting due to it not knowing where the cam was. The car also has a brand new ractetronix fuel pump in it and ran great yesterday so i doubt its that.

A couple nights back i took the car out and to my surprise it ran great with no issues and shifted at 6500 without a hitch. A couple people have mentioned grounds but I've checked all the ones around the head and frame. If someone knows or could post a diagram of where alll the grounds are I'd love to see it.

Thanks for the help

**pajeff02** · 11-10-2011, 01:44 PM

There is a ground on the back of the driver side head.

**Knight** · 11-10-2011, 01:56 PM

When you had the new fuel pump installed, did you also install a new fuel filter? Problems like this can be very hard to trace, especially when it's not a constant problem. I would go over all the basics - fuel pressure, maf, O2s and wiring, vacuum leaks, ect.. If there's any chance of getting some monitoring software, that could help.

**whitetrashbird** · 11-10-2011, 02:06 PM

I'm betting on the plug going into the cam sensor.

Dan

**redbird555** · 11-10-2011, 04:18 PM

Originally Posted by pajeff02

There is a ground on the back of the driver side head.

I checked those hwoever I know there are atleast 2 on the block and 1 on the frame I just wondering if anyone can tell me where the others on the block are?

Originally Posted by Knight

When you had the new fuel pump installed, did you also install a new fuel filter? Problems like this can be very hard to trace, especially when it's not a constant problem. I would go over all the basics - fuel pressure, maf, O2s and wiring, vacuum leaks, ect.. If there's any chance of getting some monitoring software, that could help.

I did the fuel pump install and its been working great and at the time I also installed a new fuel filter. The reason I'm shying away from fuel is because not only is the pump new but it has been working great since the install and I have no problem driving the car around town, it drives great. And once again its not like the car is sputtering or spitting it almost feels like you just shutoff half the cylinders because it has no power if you keep in the throttle eventually it will power past 5500 rpm and pull a little more smoothly as it revs to 6500 and then shifts

Originally Posted by whitetrashbird

I'm betting on the plug going into the cam sensor.
I hadnt really considered the possibility of the plug getting messed up but when I get home for thanksgiving I'm going to plug in hptuners and go from there however to the best of my knowledge the cam sensor just tells the engine where the cam is at startup and the motor should run fine once its started.. i could be wrong though?
Dan

**Firebirdjones** · 11-10-2011, 04:39 PM

Sounds like a weak pump to me. They will drive around fine, and even rev up fine, it's generally when the engine is under a real load, for a long period of time that a weak fuel pump will show it's ugly head. Simply can't keep up with fuel demands of the engine.

I wouldn't rule it out even if it's new. It wouldn't hurt, and doesn't take much time at all, to attach a fuel pressure gauge, run the gauge under the hood and tape it to the bottom of the windshield. Then go for a quick full throttle blast and watch the fuel pressure.
Only then can you really eliminate that as a possible cause. Then I would move on to other things...

**redbird555** · 11-11-2011, 03:46 AM

thanks jones, whe I get home for break next week I'm going to hook up a gauge just to make sure sure i just find it odd that it does it at a very specific time and rpm. do you think it could be a coil giving out under load? i also installed the bigger alt upgrade about 2 weeks ago also which is just the 140 amp alt from a truck, direct bolt in and now my voltage never drops below 13 but i dont really feel this has anything to do with it because the car ran fine after the install

**Firebirdjones** · 11-11-2011, 06:41 AM

Electrical gremlins are such a pain to diagnose and pinpoint if they are inconsistent. Bad coils seem to be pretty rare on these LS engines but it's always possible. I tend to start diagnosing the simple mechanical issues first, and your scenario sounds like a fuel delivery issue, which I've seen cause your symptoms many times before. Whether it be the fuel pump, kinked line, clogged filter, etc....But the fuel pressure gauge will tell you without a doubt if you are looking in the right area or not so you can eliminate that with certainty.

Just be sure to get the car to duplicate the sputter or lack of power while testing the pressure so you can see if the pressure does anything funny during that time.

The only thing that puzzles me is the issue with your tach. Why it stopped is interesting for sure. That kind of sounds like a charging issue or loose connection?

**redbird555** · 11-11-2011, 07:59 AM

The more I think about it the more I think the tach stopping is a result of of what is going on with the motor because if I let off the gas immediately when it starts sputtering the tach will work fine but if I keep into the throttle for a couple seconds while its missing the tach will quit. SO I have a feeling its more related to the fact the motor is having a misfire issue and somehow thats killing the tach. The tach to the best of my knowledge is controlled by the cam sensor so that is the next place I will look.

If it is the fuel pump I feel like I'm going to have a conniption lol because this is my 2nd ractetronix pump in 2 months the first one had a check valve issue to where it wouldnt hold a prime and now this one may have an issue. As far as a charging issue it's like I said I just installed a new alternator in there from a truck so help support the stereo system and its 140 amps so far it has been great and voltage never drops on the gauge so I doubt its that. One question tho would the fuel psi fluctuate anyway when the car starts to misfire even if the pump is working correctly simply because the rpms and engine parameters are flluctuating or should it stay at 58psi no matter if the engine is running smoothly or like crap? lol

I suppose the fuel pump issue would make more sense seeing as I can rev it to the moon in park and nothing happens but the minute I take it on the road it revs fine up to 5500 and then after that its just like someone shut down half the engine. If i give it about half throttle it drives fine its only when I got WOT i dont think a coil would work when not under load then go out when under load but who knows. When I finally get done with exams next week and get home I'll look it getting a fuel psi kit and testing it out.

**Firebirdjones** · 11-11-2011, 08:25 AM

Originally Posted by redbird555

One question tho would the fuel psi fluctuate anyway when the car starts to misfire even if the pump is working correctly simply because the rpms and engine parameters are flluctuating or should it stay at 58psi no matter if the engine is running smoothly or like crap? lol

I suppose the fuel pump issue would make more sense seeing as I can rev it to the moon in park and nothing happens but the minute I take it on the road it revs fine up to 5500 and then after that its just like someone shut down half the engine. If i give it about half throttle it drives fine its only when I got WOT i dont think a coil would work when not under load then go out when under load but who knows. When I finally get done with exams next week and get home I'll look it getting a fuel psi kit and testing it out.

Free reving in neutral to redline and sounding fine, but sputtering under load at high rpm is exactly what a weak fuel delivery will do. To put it simply, quick revs in neutral aren't enough to really strain the fuel system, it's those long, heavy load, slow rpm pulls where you find a weakness in the fuel delivery. Usually high gear (3rd gear) for a real long pull. In extreme cases it can be done in first gear (shorter period of time).
I see it on big HP applications all the time. Either the fuel pump isn't big enough or the fuel line is too small etc... Some cases it's simply the fuel pump going bad or a clog somewhere in the system.

Being an electric pump it's possible to see some fuel pressure fluctuation if the engine sputters due to lack of electrical supply,,,,but,,,,what you are going to look for under a long high rpm pull is to see that fuel pressure steadily drop under load. This generally happens before the engine sputters,,,until the pressure gets low enough to a point where the engine is starved for fuel, that's when the sputtering will start.

If you start out at 58 psi. and you start to see it decline more than 8 or 10 psi before sputtering, then there is more than likely an issue with the fuel supply. You shouldn't see more than a 4-5 psi drop in pressure under a full high rpm pull. If there is an issue, you should see the fuel pressure start dropping in the lower rpm ranges,,,say 3500 and up under full throttle, and it probably gets to a dangerous low spot in that 5,000 rpm range you are experiencing where it starts sputtering.
Easy to check this and eliminate it. The trick is having a fuel pressure line long enough to run from the fuel rail to the windshield. I use an old 8 foot long piece of R12 freon charging line. It is the same connection as your fuel rail. I then hook it to a 6" diameter 250psi gauge that I have laying around, duct tape it to the windshield and go for a run. Hope that helps.

**Firebirdjones** · 11-11-2011, 08:35 AM

Here is a picture of what I use. The fuel pressure gauge on the right is what you would find in a parts store. It's rather short, okay for working under the hood. For real testing you need to watch it while you drive. For that I use the one on the left.

That's a 120 psi. gauge (not 250 like I had thought) and it's connected to old R12 freon hose, which has the same screw on attachments as a GM fuel rail. I just run a series of the hose and make it as long as needed.

**redbird555** · 11-11-2011, 09:18 AM

thanks again jones what I just went on shmeebay and ordered a fuel pressure gauge kit for a camaro it comes with a 100psi gauge when i get it I'm going to simply get 5 feet of hose and extend it up to the windshield like you said and see what happens. Atleast at the end of this I'll have a nice stainless face fuel psi gauge under my hood lol. In all honesty if it does turn out to be a fuel pump its pretty rediculous seeing as its a rcateronix pump and the car is just a full bolt on car with an ls6 motor swap so i shouldnt be close to pushing the 600hp limit

**Firebirdjones** · 11-11-2011, 09:56 AM

Yeah Racetronix kits have a good rep ( I have a kit here I haven't installed yet) they use Walbro pumps which is in just about anything fuel injected making HP. Sometimes though I have read people having goofy issues. Hopefully it's something simple.

**redbird555** · 11-11-2011, 10:42 AM

yea I know iirc its just a walbro pump with a special sleeve and check valve installed into it to be used in the fbody. it seems some of those electric fuel pumps are hit and miss so who knows but hopefully the gauge will atleast point me in the right direction

**redbird555** · 11-11-2011, 09:08 PM

my situation took a turn for the worse tonight. Over the past couple weeks I've had a couple small instances where my car would not crank, like flat out I would turn the key and nothing would happen but on the 2nd try it would crank up just fine. I thought no big deal I'll look at it over thanksgiving break well tonight it didnt start and left me stranded. The security light is not flashing so I doubt its the VATS system but again I cant be sure I also do not hear a "click" from the starter solenoid like its even getting power. I feel its probably unrelated to the hesitation issue as this has been going on for a little longer but I'm still at a loss here

**Cutlass** · 11-12-2011, 06:42 AM

I think the tach problem and the misfire problem are related and could be caused by the Crankshaft Position Sensor or its wiring/connections.
The "no crank" problem could be a separate issue with VATs or the starter or something related to starter circuits.

**redbird555** · 11-12-2011, 09:27 AM

ya cutllass i think they are probably unrelated especially since one started before the other but who knows. I'm not comfortable with driving the car home 200 miles so im using my AAA service to have it towed home and since I wont be home for another week I'm going to have one of the ls shops ddown there take a look at it and do some data logging they seem to think it might be a loose ground causing both issues. who knows but hopefully they'll get it sorted out

**redbird555** · 11-23-2011, 03:34 AM

just an update guys but im afraid its not very good. The car is over at an ls shop where they have been looking at it on the dyno for a few days. What we have gathered is that the car will misfire every time at 5900 rpm the graph dips down about 10 hp with each misfire but will then rebound up to its previous reading before it misfires again.

The odd thing is that when the widebands are plugged into both sides of the engine, both banks go lean the same amount, misfire at the exact same time, and at the exact same rpm every time. The problem never fluctuates in the rpm band. So we're thinking that the chances of it being a bad coil, plug, or wire if very slim seeing that it happens all the time at the same time. Were going to try swapping coils with a known good set just to be sure and the fuel pressure was checked and holding good at 59psi up top.

They're thinking maybe a bad pcm since the problem is so exact, if it were a fubared ground or power wire or an ignition problem I'd think that it would be having issues at varying rpm points or on one bank.

**Firebirdjones** · 11-23-2011, 06:03 AM

In that case I'd check valve spring pressure as well, to be sure. As they get weak, high rpm valve float is usually the first sign. Real easy to pull a valve cover and use a handheld spring tester for a quick reading. Also gives you a chance to look at the springs to make sure there isn't a broken spring or two.

**redbird555** · 11-23-2011, 10:14 AM

thanks jones were going to try a pcm swap since theres an extra around and see what happens if not then that will likely be the next step. I'd think a broken valve spring would cause issues everywhere not just up top like that though? The engine sound mechanically fine, no noises or hiccups until that rpm, also the springs are only 6 months old haha and its just a stock ls6 cam so it shouldnt be too much of a stress on them.