Pulsing Hesitation at Constant Throttle

I have something strange and very annoying going on with my car. A little background info. first - I have a 2005 LGT MT Limited w/ aftermarket turbo (Dom 2.5R), all necessary supporing fuel mods, FMIC, Cobb Intake, and of course a full catback aftermarket exhaust. The car was dyno tuned Dec. 18th, 2008 on a Mustang Dyno using a Cobb AP V1.

A couple of weeks ago, I started noticing that my car was experiencing pulses of hesitation or back pressure at constant throttle. It doesn't seem to matter what RPM or what gear I'm in, when I have my throttle depressed at a constant pressure, I notice this hesistation or stutter, kind of like engine braking...When this happens my AFRs jump up like my car is running too lean for these brief and repeated pulses. It's becoming very annoying.

I've had my mechanic look over my car thoroughly, and he did find that my Perrin EL Headers were leaking some, so he fixed them. He checked for any other possible exhaust or vacuum (boost) leaks...none to be found. Even after the header leak was fixed, I still notice the problem, almost more prevelant but hard to say for sure. The only thing we can think of is that it must be something to do with the tune. I'm waiting for a little bit warmer weather to log my tuner some data from my car to see if he can smooth this out. I find it odd that I didn't notice this right after the tune, and it wasn't until just about two weeks ago that I started to notice these symptoms. No doubt the header leak was there during the tune, but even with this fixed, it doesn't seem to make a difference.

Anybody have an experience with a similar symptom to this or have any idea what could be causing it? I'd appreciate any input members may have......

Your BOV is full recirc?

I've got a Forge Diverter valve.

So I assume it is in full recirc anytime the engine is producing vacuum? And, it recircs back into the intake?

Moved to Tuning Forum. Interested to hear what the resolution is to this issue.

Originally Posted by Subietonic

Moved to Tuning Forum. Interested to hear what the resolution is to this issue.

Thanks for getting me to the right place.

Originally Posted by GTTuner

So I assume it is in full recirc anytime the engine is producing vacuum? And, it recircs back into the intake?

Yes...it should be...and dumping excess charged air back into the intake.

My opinion is one of two things;

A torn inlet hose or FMIC piping leak, or the diverter valve is dumping massive amounts of air back into the loop. I know it dumps after the MAF, but when I was tuning my BOV I had the exact same condition when it was constantly on full recirc when cruising. Try disconnecting the vacuum line at your diverter valve and drive it to see if your surge is gone. Just don't get into any major boost with the valve disconnected.
You may want to try and clean the MAF if none of the above resolves it.

Are you still running your rear o2 sensor in the stock position? I believe the rear o2 is used to monitor cruise a/f ratios...i am guessing it could cause some fueling issues under cruise conditions if the sensor was disconnected but that would likely trigger a CEL.
Some folks use the rear o2 bung for their wideband, if you do this you need to send a fake narrow band signal to the ecu.

Originally Posted by GTTuner

My opinion is one of two things;

A torn inlet hose or FMIC piping leak, or the diverter valve is dumping massive amounts of air back into the loop. I know it dumps after the MAF, but when I was tuning my BOV I had the exact same condition when it was constantly on full recirc when cruising. Try disconnecting the vacuum line at your diverter valve and drive it to see if your surge is gone. Just don't get into any major boost with the valve disconnected.
You may want to try and clean the MAF if none of the above resolves it.

I mentioned in the original post that I noticed the symptom at constant throttle pressure, not really under acceleration (sometimes I notice a slight hesitation at the onset of acceleration). I'm still hitting my boost target and the valve seems to be releasing at the point it should be. Also, I seem to notice the most dramatic hesitation pulses, under very light throttle pressure or after I accelerate a little bit then back off the throttle and maintain constant pressure. Do you still think the above could be the case? I'm pretty confident all my hoses are well connected and that there are no tears, and that my FMIC piping is secure and not leaking. I've looked these over carefully, and so has my mechanic who is very familiar with Subarus and their modifications...not sure on the diverter valve disconnect though?....I suppose it could be doing what you mentioned. I could try that and make sure the MAF is very clean. Would you recommend I try this before logging my tuner data? Also, what would be the fix assuming disconnecting the vacuum line to the diverter valve seems to fix the problem?

Thanks for you input.

Holy crap, you just described what my car was doing to a "T"!

I seem to notice the most dramatic hesitation pulses, under very light throttle pressure or after I accelerate a little bit then back off the throttle and maintain constant pressure

I had my BOV so tight it was stalling my turbo when I let off the gas, so I loosened it up a bit, soft spring and full open under 5" of vacuum. From what I understand an diverter valve is either fully open or fully closed. If U disconnect the valve to keep it shut, and what you described is gone, you may need to replace the diverter valve with a "tuneable" BOV. When I installed my FMIC I was looking into a ProCharger diverter valve but the people at ProCharger did not recomend it for my application.

I would def recomend checking your turbo inlet hose, they are notorious for tearing, then bypassing that diverter and see what happens.

Keep me posted.

Originally Posted by GTTuner

Holy crap, you just described what my car was doing to a "T"!



I had my BOV so tight it was stalling my turbo when I let off the gas, so I loosened it up a bit, soft spring and full open under 5" of vacuum. From what I understand an diverter valve is either fully open or fully closed. If U disconnect the valve to keep it shut, and what you described is gone, you may need to replace the diverter valve with a "tuneable" BOV. When I installed my FMIC I was looking into a ProCharger diverter valve but the people at ProCharger did not recomend it for my application.

I would def recomend checking your turbo inlet hose, they are notorious for tearing, then bypassing that diverter and see what happens.

Keep me posted.

Interesting. I never would have thought it could be the diverter valve. I'll look into it and tell you what I determine.

subscribed

I have a similar surging problem after installing my FMIC. I'm curious to follow this thread and perhaps lend a hand since I have the same problem. My surging seems worse when I am at very light throttle, pulling about 16-20 inches of vacuum. Once I am heavy enough on the throttle that I reach 10 inches of vacuum, the surging goes away.

I've logged during this surging and I have not found any obvious problems, fuel trims are working properly in CL and AFR is switching just above and below 14.7. Timing is steady.

I have the stock BOV, which leaks a LOT of air back into the inlet hose during idle and very high vacuum situations.

I have some other fueling issues I need to get fully sorted before I can go much further on my car. But if I can help, let me know.

Jeremy

lean a/f is a mechanical issue. I guarentee your tune is very rich and conservative. Check for vac leaks/intake/inlet/fmic leaks. Is your fuel pump a hacked fit version? Mine failed some of these symptoms you are describing are similar. Is it still hovering around 14.7 during closed loop operations, if it randomly is spiking under closed loop situations its a fueling issue im betting or a leak. Something mechanical has changed.

Id start with cleaning your maf and pressurizing the system to check for any leaks first. Go from there.