bucking and resistance under boost like the rev limiter or ASR at 4K-5K RPM - PeachParts Mercedes-Benz Forum

#1 06-04-2005, 04:24 AM

I have a fairly general question about a problem with "bucking" /resistance under boost that feels just like I am hitting the rev-limiter when the car is in park.

Car: MB C36 AMG twin turbo
Rev Limit: 6400 - 7000 (redline is 6400 but it goes to 7K)
Max Boost: 8psi
Turbos: Garret T25
Engine: Inline 3.6L 6 cylinder

Symptom: Formerly at 4000 RPM now at 5000 RPM especially when the car has not warmed up it would hit a hard limiter i.e. bounce off the limit. A/F ratio leans way out. And letting off the gas slightly or even keeping the same pressure (i.e. not depressing the peddle further) induces a BOV dump. I have not idea why it is doing that. I don't push it because it feels awful but I think that I might be able to push it past that point. Also after a long while when the car is sort of hot it goes away in a sort of random fashion. The computer sets the rev limit at 4K RPM when in park but that shouldn't be an issue when moving + the problem is now at 5K rpm. I blew a hole in the intake hose (which was old and not designed for turbos) and when air was leaking out I wasn't getting boost at all even under full throttle and the "limiter" went away. It was also running rather rich all the time when this happened. Everytime I have blown a hose off I feel that it was due to hitting this resistance and bouncing off it several times under high boost and being lean. Never had the engine knock or ping from it though. It was after changing the hose back that the limit "moved" from 4K to 5K. I still have boost at the limiter and it feels really aweful like something is trying to blow (I think that is why the intake blew off one time and created a hole another time). I have also read that overboosting (which I don't think is the case) can cause this sort of behavior but that doesn't make sense because when the car gets more boost and faster due to the air not leaking out the limit moved up not down. I have heard of BOV leaking boost but I have never heard of them causing the car to hit a rev limit. I am wondering if I should set the BOV to open under more resistance or if there is something else I should check. The other thing that I am thinking about is if somehow the rev-limiter is being triggered by the pressure e.g. the transmission is engaging/disengaging due to positive pressure rather than vacuum. Unfortunately no data logging.

300EVIL · #2 06-05-2005, 02:18 PM

While I don't have much "hands on" experience, my educated opinion would be to look at the valvetrain. This sounds like an exhaust and or intake reversion problem which would spawn from a combination of valve overlap, bad spring harmonics and boost pressure. My guess would be that the valves are bouncing off their seats around 5K and is killing your power. The problem may be disappearing when your engine warms up because the harmonics of the springs change as they warm up. I had my cam reground to get rid of the overlap as well as beefed up the springs to "hopefully" eliminate the possibility of a reversion problem at high rpm and boost levels. Of course, this problem could be a number of different things but, this is what it sounds like to me. I would suggest turning down the boost by a couple psi and see if the problem still exists.
Good Luck!
Adam

TheV12pwr · #3 06-05-2005, 02:45 PM

I would check the throtle position sensor...
I had 300ZX TT that will not go over 4000RPM hesitated and no power...
it was lean on gas because of the TPS.

300EVIL · #4 06-05-2005, 03:35 PM

It could very well be the TPS too. My M104 has a bad throttle body assembly. I have done a lot of research on these TB's and they all seem to go bad eventually. I just hate the idea of having to spring $1K for the damn thing. I assume you have already checked for fault codes to see if this could be the problem?

As for the vacuum modulator on the trans, there is a mod you need to do that adjusts the vacuum/pressure input so the trans can properly recognize load. The mod is done with orifices and a needle valve to properly interface the new pressurized signal. I will find the diagram and post it sometime this week.

#5 06-05-2005, 08:29 PM

Well last time codes where pulled there where ton of them due to a hole in the intake but I will have them pulled again and have the TPS checked.

Quote:

Originally Posted by 300EVIL

As for the vacuum modulator on the trans, there is a mod you need to do that adjusts the vacuum/pressure input so the trans can properly recognize load. The mod is done with orifices and a needle valve to properly interface the new pressurized signal. I will find the diagram and post it sometime this week.

If you could find the mod that would be great. It shifts rather rough sometimes due to this as well. I heated it up yesterday and it never hesistated. It takes quite a while to heat it to that point so I am sort of wondering if it might be a sensor e.g. factory O2 or MAF not heating up right away.

Next problem: It is flooded when I depress the button on the floor. It can be running lean at partial throttle and pinging away but depress the button and I'm seeing >9:1 A/F ratios. It must be the factory system because the auxillary fuel computer goes by boost pressure and RPM.

300EVIL · #6 06-22-2005, 04:35 PM

Here's the vacuum modulator mod.

#7 06-22-2005, 05:23 PM

Quote:

Originally Posted by 300EVIL

Here's the vacuum modulator mod.

Hey, thanks a lot! My dad actually just adjusted the knob on the side of the transmission and it works great now. He also adjusted the bowden cable. I am wondering if anything else depends on manifold pressure and needs to be adjusted.

Anyway here is part of 2 emails that I sent to the previous owner. He's a really nice guy and knows a lot about the car so I am hoping that he can help me. If this triggers any ideas let me know. It doesn't have any codes. How does one go about checking the throttle position sensor? It is fine if you increase the RPMS slowly and I think you can force it through. Under full boost but not depressing the button it will hit buck, backfire, and downshift sometimes. I may just be screwed and will have to get my ECU re-mapped to keep a lower A/F ratio say 12.5 ($$$) and figure out how to make it respond quicker or replace the stock system with a standalone ignition/injection computer.

From email 1:

Quote:

I am just now getting back into my car. It has been in the shop quite a bit with them replacing things and re-attaching lines and hoses that I blew off/up. I got new spark plugs, brakes, rotors and other stuff that I can't remember. It is going back into the shop on Thursday to see if they can figure out the bucking/resistance issue. I have the EGT gauge sitting in the car but I think I am going to go with this system that combines wideband A/F, EGT, boost, RPM, and Lambda instead: http://www.zeitronix.com/Products/zt2/zt2.htm I need data logging and a wideband A/F meter because I can't watch RPMs, boost, EGT, and A/F all at the same time + this unit integrates everything into a small package that will fit nicely into the spot where the gauges are now. It is also an LCD so it looks better sitting below the radio than the gauges

I found a place about an hour from my house that sells 100 and 104 octane racing fuel so I think I'm going to be mixing that with the regular fuel and running straight 104 on the weekend. Once I can get the bucking/rev limit thing ironed out and get the fuel ratios set up properly using the wideband I'm going to raise the boost. I have gotten to drive it a few times where the car was working pretty well. However, it always goes back to bucking/lean/rev limiter conditions so unfortunately it still isn't drivable. I do have hope though because when that problem went away it was awesome. I have been able to eliminate the bucking/resistance problem in two different ways.

1. I have destroyed two rubber sleeves (the one that goes over the intake and the plastic 3.6L tube) and when they are leaking the boost only goes up to about atmospheric pressure. It runs great then because there is no bucking and no resistance and it just really tears through the RPM ranges.

2. another time I basically turned up the fuel to the max at the point where I was encountering resistance and it went away for a while. I was getting full boost of 7.25psi and it really ran great then. Same thing very little resistance and just tearing through the RPM ranges. It sounds awesome when you can drive it like that. Going through an underpass at full throttle it sounds like a Ferrari. Unfortunately the problem came back after a few hours of driving. I think the car's computer adapted and started leaning it way out again.

From email 2

Quote:

I don't know much about how the system works in closed loop mode but
let me run this by you. The O2 sensor is right before the cat right? If the
car's computer uses this sensor to keep a 14.6 ratio then the MF2 (auxillary fuel injectors computer) is only useful when in open loop or full throttle mode or when the stock system cannot maintain 14.6. I am thinking that the bucking/rev limit/lean condition is related to this closed loop mode because I don't have the bucking/rev limit/backfire/lean condition under any of the
following circumstances:

1. when the engine is cold i.e. when it is running default maps because the
O2 sensor hasn't warmed up
2. under full throttle when it is running in open loop mode.
3. I have very low boost and very slow buildup of boost. in this case I
would say that no lean condition is going to develop because the stock
system can respond to the slow increase in boost and also doesn't need to
add that much fuel anyway.

Do you think I should replace the stock O2 sensor just to make sure that the
slowness of the car's computer to add fuel i.e. eliminate a lean condition
is not due to lazy O2 sensor or is the stock system not going to be able to
keep up with the rapid change in A/F ratios? I think boosting the fuel
temporarily worked because it was such a drastic change that it took a
little time for the stock computer to re-do a fuel map so I was actually
able to run rich in closed loop mode. Pinging/Detonation is occurring
because the stock system is not allowing more fuel through not because the
ERL isn't supplying it i.e. the stock ECU is allowing the mixture to get
really lean i.e. >15-16 for ~ half a second under partial throttle before
allowing more fuel. I can tell because I will have partial throttle with
increasing boost but still < atmospheric pressure and it is totally lean but
after about half a second it adds more fuel. In the mean time though if you
do manage to get up reasonable boost during that half a second you can get
pinging/detonation. Is there any way of fooling the stock computer to allow
more fuel under closed loop? Or at least making it so that it responds quick
enough such that it actually does keep a 14.6 all the time under throttle
rather than allowing these lean spots to develop when throttle is starting
to be applied? Maybe by switching inputs to the stock system around e.g.
taking the A/F reading from the narrowband output from my new A/F meter
instead of from the factory A/F sensor or taking the reading before the
extra fuel is injected AND/OR getting a better/faster O2 sensor? If I can at
least make sure that it is quick enough to maintain 14.6 I can use high
octane fuel to eliminate detonation/pinging. I can't do anything if it isn't
quick enough to allow more fuel through when it is needed. I very rarely get
pinging/detonation under full throttle as it usually runs way rich under full throttle. The RPM, boost, and A/F needles move around so fast under full throttle that I can't keep track of them to identify where/if a lean condition is occurring. I am going to use the data logging capability of this new wideband A/F meter in conjunction with the boost logs to find out exactly what the A/F ratios are under full throttle at the various RPM ranges.

#8 06-25-2005, 10:51 PM

All right I fixed it at least it has been fixed for the past 5 hours of hard driving. By disconnecting the two O2 sensors (before and after the CAT) I am able to use the auxillary computer to run whatever A/F ratios I want. So I tuned it to run a <12 A/F ratio at any RPM and at any boost and it runs awesome. Bucking/resistance is gone. I don't know what the long term ramifactions of not connecting the O2 sensors are but so far no check engine light and it is running great. The MB computer star diagnostic or whatever it is says that the voltage running to the O2 sensor heater is too low and I think that's the only thing. If I do in fact have to re-connect them then I am going to put a switch inline that will allow me to turn them off for when I want to go. Another option would be to figure out how A/F to volt translates and put a resistor as needed to decrease the voltage i.e. run richer all the time.