Terrible Hesitation/Stutter

[wookieneck]

Researched this a lot and found nothing. Hopefully someone can save me......

When accelerating I am getting horrible hesitation, which I assumed was fuel cut. I had the missing link, so I took it off and put on the FCD. Same thing...

Running 4 psi it is somewhat noticeable, when I turn on the boost controller and run 7psi it bucks hard.

When it happens the A/F goes lean. Normal bounce, normal bounce, RED (big buck)
If I ease into the throttle it does not seem to be as bad, but when I nail the throttle it always bucks hard. The Fuel pressure gauge is consistent when the bucking occurs, in fact the only gauge that tells me anything is the AF.

In addition to changing out the ML for the FCD I have replaced the O2 sensor, the coil, the plugs, the plugwires, the injectors and the MaP sensor. Only thing I havent replaced is the computer and the TPS, oh and the cam sensor.

WTF?

 

[BigRand]

What kind of FCD are you using. The symtoms your describing sound like an FCD problem. I'd say make sure its wired correctly, and if you got the hahn fcd, you might consider replacing it.

 

[wookieneck]

The one that Paul sells here on the classifieds.
I start at 4.7 volts like it says, and get lots of what seems to be fuel cut.
adjust it down to 4.5, fuel cut
4.2 a little less fuel cut but it doesnt pull near as hard when its not cutting.
3.9 same
3.7 still some fuel cut but it runs lean constantly and runs like crap otherwise
Havent taken it any further down than that on the FCD, I think I might be able to eliminate it with that but I'm starving it for fuel.

 

[4UH8ERS]

sure sounds like fuel cut to me. if your sure the FCD is installed correctly and is working properly then you might want to change the map sensor you have all together. It definately seems like its a MAP problem. did you solder the wires or just crimp them together?

ohh and btw why dont you test it by pressurizing the system while not driving and make sure when positive manifold pressure is applied that the fcd is working properly. just an idea

 

[VelocitàPaola]

That's odd. Why don't you go over the wiring again, and post exactly how you installed it for us to review. You shouldn't be experiencing behavior like that at all.

 

[Blitzeclips]

Check the feul pump/filter. Fuel lines. FMU. Anything fuel related may yield the culprit here..

 

[wookieneck]

Double checked the wiring, its crimped for the time being. I will get around to soldering it after I sort this issue out.

I bought a new map yesterday and put it on, no difference.

I made a jig to pressurize just the map, during calibration. Been running it up to 10psi and only see a .05 volt increase over ambient.

Fuel system- Walbro 255 less than two years old, Oem filter less than 1 year old, oem line to the rail with a pressure sensor right before the rail.
Out of the rail with 4an stainless to the SFMU with gauge on it, and stainless return line.
I had the FMU, and had this problem. Yanked it and the factory FPR off and installed the SFMU, no difference.
12:1 disc with the stiff spring, 40 psi at an idle, It rises consistently and does not fluctuate when the studder occurs

 

[cs82685]

12:1 disc with the stiff spring, 40 psi at an idle, It rises consistently and does not fluctuate when the studder occurs

I'm assuming this is on stock injectors. If so, that's on the low side for idle fp. Shoot for 47-55. Although that probably isn't your problem considering it also did the same with just fmu. It is one of the simplest things to change though so I'd say it's worth a shot.

 

[wookieneck]

Its probably closer to 45 psi on the feed side of the rail, at speed, slightly lower at idle. I will try to crank it up though, thanks for that thought.

Just swapped out the stock injectors for 270cc accels, so not a huge jump. 210 is factory if I remember correctly, but its late and my brain is like goo.

 

[cs82685]

Its probably closer to 45 psi on the feed side of the rail, at speed, slightly lower at idle. I will try to crank it up though, thanks for that thought.

Just swapped out the stock injectors for 270cc accels, so not a huge jump. 210 is factory if I remember correctly, but its late and my brain is like goo.

I believe stocks are 235cc, but I could be mistaken

 

[BigRand]

Double checked the wiring, its crimped for the time being. I will get around to soldering it after I sort this issue out.

I bought a new map yesterday and put it on, no difference.

I made a jig to pressurize just the map, during calibration. Been running it up to 10psi and only see a .05 volt increase over ambient.

Fuel system- Walbro 255 less than two years old, Oem filter less than 1 year old, oem line to the rail with a pressure sensor right before the rail.
Out of the rail with 4an stainless to the SFMU with gauge on it, and stainless return line.
I had the FMU, and had this problem. Yanked it and the factory FPR off and installed the SFMU, no difference.
12:1 disc with the stiff spring, 40 psi at an idle, It rises consistently and does not fluctuate when the studder occurs

Okay so I could be off on this because I'm still not back to normal from last night, but I thought the feed line was supposed to to the sfmu, then to the fuel rail......I'll edit this in later on.

 

[wookieneck]

No it goes between rail and return. If it was between the pump and the rail you would only hold pressure between between the pump and FMU, the rail would only get the bleed off and that would make for unhappy injectors.

I did crank the static pressure up and the bucking seems to be lessened. I will try to bring the voltage down on the FCD and see if I am in the ballpark.

The thing that kills me about the fuel pressure is the difference across the rail. I have a gauge on the feed side, the electronic autometer with the sender, and a liquid filled on the return side of the rail.
There is about a 10psi difference between the readings, I would expect a maybe 2 or 3 lb variance but 10psi seems excessive. I might add a mechanical next to the sender so I have a side by side comparison under the hood.

ON the subject of fuel pressure- what is everyone carrying around 7psi of boost?

 

[wookieneck]

dialed the static pressure down a bit and dialed the FCD down to 3.9.

It pulls hard but still cuts out.
If I ease into it, the thing is smooth and fast but if I punch it down I about get thrown out of the car with the bucking.

Anyone have this issue before?

 

[Locke]

how quickly does the regulator react to the increased fuel demand? when you open the throttle, how low does the pressure drop, and how long until it returns to normal?

also how is your ignition doing? plugs read OK?

 

[wookieneck]

There is never a drop in fuel pressure below static, it instantly climbs with the boost.

Plugs read? They are fine I guess, not fouled- looks like they are burning clean.

If I remove boost from the equation entirely and accelerate I still get some hesitation. So I am wondering if it is computer related.
The computer and TPS are the only electronic things that have not been replaced...

 

[wookieneck]

I am at my Wit's end on this. I broke down and tried to take it to a local speed shop which is no more apparently.
Tried to read the codes, EGR is the only thing its throwing. Which seems fine that the EGR is gone.
Im looking for a computer, if that doesnt work I am dousing the whole thing it fuel and moving on to a different project.