yitzac1990

Wasn't sure where to start this thread, since its for a silverado engine, but still a gen 3 LS based engine haha.

I am getting misfires above 4k rpm in my lm7 swapped drift car. Been having the problems for a while, but been really busy and just recently getting time to try and narrow it down. Here is a timeline of what I have done/parts replaced:

Has relatively new injectors already (replaced from an unrelated fuel problem I had a while ago)
external walbro 255lph fuel pump
AEM FPR (set at 55 psi with no vacuum)
aftermarket billet fuel rails
stock truck intake manifold
4" aluminum intake pipe, with couple year old MAF

Started having the misifring problem, but only above 4k RPM

replaced all coils with MSD street fire coils
replaced plug wires with MSD wires
replaced plugs with NGK Iridium
added spark plug boot heat protectors/socks (left the OEM metal induction blockers on)

Car seemed to do fine for a little bit, but again would misfire after 4k RPM just like before. Thats how it has been for a very long time while I didnt have time or money to mess with it. Now that I had some time:

replaced spark plugs with factory ACDelco spark plugs (old spark plugs looked fine when taken out)

After I let it warm up, I did a WOT pull and it ran perfect!!! Then I did a second WOT pull and back to misfiring above 4k prm.....same with a 3rd pull. ECU not throwing codes. My buddy with a Matco Maximus diagnostic tool plugged in and we went for a ride, and said it had some random little misfires at idle and driving normal, and then above 4k rpm it was going crazy with misfires everywhere.

I tried asking some car friends and I got all sorts of responses...coils crappy, vacuum leak at manifold, bad cam/crank sensors, bad plugs, etc.....

Does anyone have any idea? I am thinking about replacing the cam and crank sensors, because I found a thread online saying it would fix random misfires in the mid-2k rpm range. Might that help me with my 4k+ rpm range?

G Atsma

Sounds like it's starving for fuel. Since it's not mentioned, have you checked the fuel filter?

yitzac1990

oh i did forget to mention, that even though I haven't actually checked the fuel filter, i do have a fuel pressure gauge and shows its not losing pressure all through the RPM band. but i could change out the filter for laughs anyways since its so simple and cheap I guess.

G Atsma

Good idea, but with full pressure at all times the filter might not be the issue.

yitzac1990

does anyone else have any ideas? not in much of a rush, but would like to get some parts on order if needed

G Atsma

Just to rehash- you are SURE you are getting a FULL 55psi at ALL times?? Otherwise I would suspect the FPR getting a little wonky at times(obviously not all the time).

yitzac1990

I can do another pull when I get some more time this weekend maybe, but yeah, I didnt notice the fuel pressure dropping off at all throughout the RPM range.

nine8ss

iTrader: (5)

I would go with faulty crank sensor or an issue with your reluctor wheel based on this.

yitzac1990

im curious, what might cause an issue with the reluctor wheel? i figure if something was floating around in there big enough to damage the wheel, it would probably cause catastrophic engine failure or something too haha.

But i doubt its the wheel, because like I said, the first pull after I changed the plugs (could have been a coincidence) was perfect, no misfiring at all, all the way up to redline. i guess a faulty sensor could give intermittent problems.

G Atsma

Yeah, with fuel pressure constantly high enough, it leaves electrical as an issue, and the above would do it. The intermittent nature also speaks to this.

nine8ss

iTrader: (5)

Thats true, the 1 good pull kinda negates the wheel issue. Pull the sensor and look for any contact marks or signs of wear. I remember reading a thread a while back where someone had this same type of issue and he found a bent tooth on the wheel. I will see if I can find the thread and link it.

yitzac1990

i guess I will go ahead and replace cam and crank sensors for good measure. good insurance anyways, since i didn't replace them when i got the engine, and unsure of previous owners use (and mileage, really) of the engine.

I would think a bent tooth on the wheel would cause consistent problems, agreed? man I hope its not the wheel!

Not sure when I will have time to order and install the sensors, but I will try to keep the thread updated when I do so.

G Atsma

A bent tooth or teeth would show on every revolution, where a bad sensor could be intermittent.

joecar

Did this just suddenly start happening...?

Did you verify that the tune does not have rev limiters set to 4000 rpm (sounds obvious, but don't assume anything)...?

yitzac1990

its been happening for a while, I forget how "sudden" it happened, or how it came about. The tune is the same as it has been since day 1, which is basically just a stock truck ECU/tune with VATS removed.

cookseyb

Knock sensors pulling timing. Make sure your harmonic balancer is in good shape..

yitzac1990

the balancer should be good, as it hasn't changed since I got the engine, but will do a visual inspection of it tonight. I am planning on changing out the cam/crank sensors this evening.

If the cam/crank sensors don't improve engine performance, I will go with knock sensors next.

Would some kind of vacuum leak be a possible problem? I figure at WOT there isnt any vacuum anyways, but a couple of my buddies mentioned it may be that. However when spraying starter fluid all around the intake manifold at idle, there were no sudden jumps in RPM, which leads me to believe there aren't any leaks anyways.

pantera_efi

iTrader: (1)

Hi Zac, I agree with the Fuel Quality "tech".

The fact of a new fuel system requirement is stated.

The fuel pump WILL pump BOTH Fuel/Air with a report of proper pressure.
The fuel could be full of Air Bubbles ?
The best test is to remove the fuel line from the rail AND safely fill a clear gallon container observing the fuel flow stream.
There should be FEW air bubbles.

The NEXT test is to examine your fuel system design needed for a "drift" car.

You should have a "make-up" tank that feeds the High Pressure pump ?

Lance

yitzac1990

There is no surge tank, as I am not really at that level of speed in my drifting yet. However for my next build ill be using a better fuel cell with slosh foam. I can do the fuel test no problem and check out what its pumping, but the fuel inlet in the tank is in the back center of the bottom, so it should just be picking up fuel, especially on straight-line WOT runs.

I see you are from SNA...I was just there yesterday! haha

yitzac1990

little update:

meant to get back here earlier, but been real busy with work. swapped in the cam and crank sensors. seemed to do better, but it did stutter a bit in the top end still. although this time it was "smaller" stutters and was much more intermittent. I would be at WOT, getting up there in RPM, then would stutter for a second, keep going, stutter again, etc. And it happened at different RPMs each time it did happen, but again always above 4k.

Will try the fuel flow test and check out the filters.

One question I do have actually: can a "bad" fuel pump still be able to keep pressure correct, but still not be pumping "correctly"? The reason I ask is because a while ago I had a leak in my fuel cell. I got some permatex sealant that was supposedly gasoline resistant. I sealed the inside of my fuel cell on the seams with this stuff, stopped leaking for a bit. Then the car started running funny. I found out that all the sealant came off the fuel cell, and went through the fuel pump, into my injectors and ruined the injectors. That is why I had to buy new injectors a while back. and I didn't put that sealant back in the fuel cell, but I left the fuel pump and didn't replace it. I couldn't tell from visual inspection if anything was wrong with the internals. Maybe the pump is damaged and even though its pressurizing good, its not pumping "correctly"??