So sorry if this turns out to be a long read or belongs in a different section. I have been fighting with my new 408 for over a year now with the same problem. The car has been to my tuner 2 different times in an attempt to get the tune sorted out and both times the car would not make power past 3000 RPM and fall flat on its face, he also told me that the car was trying to get more air/ fuel than it actually needed and timing was off from where it needed to be. So after the first go around I pulled to front cover off the motor to verify that timing on the chain was correct and it was. He also mentioned that he had seen this very scenario on a 408 similar to mine and told me that the pushrods are too long and holding the valves open slightly too long. So I measured my pushrods and ended up deciding I was going to go with a slightly smaller push rod compared to what my builder had. This didn't fix the problem either, the second go around on the dyno yielded the same results as the first time. He said he deactivated the MAF to see if speed density had different readings and the car still did the same thing. After having trailered the car back to my house and tore into it more. I have checked compression on each cylinder, fuel pressure to the rails, leak down test, changed plugs for fresh ones before the second dyno. My last concern is could this be a MAF problem? It is a TSP 100mm mas air meter, could my sensor be bad? The car was throwing P0103 even with the MAF supposedly deactivated and when I unplug the MAF the car runs worse than before. Is there a possibility im chasing a bad valve spring or something else internally? Or am I at the point of needing to tear the motor back down to the block to check my builders work? Any help or guidance is appreciated being I spend a lot of time and money having this built and fighting with it for this long.

408 iron block
Stage 3 LS6 heads
Custom grind cam (don't remember the specs being its been a while)
Racetronix pump with harness upgrade
42lb injectors
fast 102 manifold/throttle body
TSP 100mm MAF
Comp 11.5:1

 

Where did you get this 408?

 

P0103 means the MAF is deactivated. The IAT is built into that MAF if I'm correct so when you unplug it its not readinfg IAT so it runs worse.

Any other codes? Misfire, cam or crank sensors? Misfires will not be detected in speed denity mode BTW.

Does it break up over 3 grand or does it sound "normal"?

I would take a few springs out and have them checked for seat and open pressure. Could be valve float although it ususally happens at a higher RPM and predominantly on FI motors.

Could be something as odd as the reluctor not pressed onto the crank in the right position, which is hard to check. Or it could be the reluctor is coming apart as I'm sure it's a 24x. Either way the crank would have to come out to check. Thats a last resort.

 

The car sounds healthy when the RPM's get up, no other codes have popped up. The car does like to backfire when you let out of the throttle. I have replaced the crank sensors before the second dyno run for a piece of mind. The tuner told me he could not see any valve float or clutch slipping on the graph. The car seems to idle fine and low end is ok but you can really feel the lack of power in the midrange and up top.

 

Was built by a friends that dad that is a machinist and engine builder.

 

You say you have checked fuel pressure at the rail, but have you checked it at the rail under heavy load (either dyno or with the gauge taped to the windshield on the street)?

Sure does sound like a fuel issue.

If not, what kind of exhaust is on this thing and have you checked the pipes/mufflers/ and cats (if it has them) for obstruction of flow?

 

Haven't checked under load yet, I have a gauge I could hook up and check with.

The exhaust is 1-7/8 headers into a custom 4" exhaust with a flow master merge collector with no cats.

 

Got the dyno graph?

 

I'll ask him if he can send me the graph.

 

Are you using the same fuel system as your last setup?

 

Find a new tuner

 

Is the rpm at which it goes wrong the same under WOT and slowly accelerating?

 

Same setup for both visits

 

Both scenarios it acts up around 3000-3400 RPM, regardless of how I am driving it. Always back fires when I let out of the throttle and as it revs up my wideband is very erratic. It gets random lean spikes at cruising speed.

 

Is the cam sensor in the timing cover?

 

The AFR hunting while part throttle driving is normal. It's usually between 14.1:1 and 16.1:1 while I'm cruising. You can command it to hold a set afr with hp tuners and it will, but the hunting is because the pcm is reading the data stream and constantly adjusting timing and fuel.

 

Back side of block behind the manifold.

 

definitely check the fuel pressure under load. i had a similar issue with my car. it would make power up to about 3300 and then lean out and quit making power. found out my pump took a ****, so i replaced with a walboro, went back and had the same issue again, but this time the pump blew the feed hose in the tank out. replaced that and everything was good after that.

 

Well I know what im doing this weekend. Seems like this might be the last thing to try before taking a look at the reluctor wheel.

 

The reason I asked about the fuel system is; when I replaced my pump I didn't get the flex hose crimped on tight enough. When I took my car to get tuned, I got a call from them stating that the car kept going lean @ 4K and up and would obviously lose power. They said it didn't matter what they did as far as the tune went, it must be mechanical. So I installed an electric fuel pressure gauge and sure as ****, 4K = fuel pressure drop. I swore up and down that those crimp clamps were tight on the flex hose but I dropped the tank anyway just to check. When I was looking at the clamps they looked tight, felt tight but I used an actual crimp tool and cranked on them so I would know that if there was still an issue, that wasn't it. After I put it back together I did a few WOT blasts and the pressure stayed firm. Problem solved.