Engine bucking/fuel ratio going lean

[Flint]

Really need some advice here. 1998 GS-T, light mods only including Hahn Racecraft intercooler and intake piping, 3-inch cat back exhaust, Stillen Ram air intake and greddy Type S blow off valve. Everything else is stock. All of a sudden last week boost is spiking higher than normal to 17-18 PSI, holding for 2-3 seconds then dropping back to 14.5 where it should be, at the same time while the boost is too high and even when it returns to normal, my fuel ratio shows lean, and the engine stutters almost sounds like a machine gun rapid firing but obviously not that loud. The only thing I have done recently to the car was a timing belt change, new valve cover gasket, and the stock oxygen sensor failed which I replaced with a Bosch unit. This was done a few months back with no problems showing until last week. Sorry this is long, just trying to be specific. The problem does not occur and low rpm, low boost, only when full throttle and boost builds to full. This problem is killing me, can't figure it out!

 

[TunaTalon]

Going lean while on boost is consistent with a failure of the FPR to raise fuel pressure 1:1 with boost. For example if the vacuum line came off of the FPR.

Can you log fuel pressure and manifold pressure?

 

[Flint]

Good suggestion, all connections/lines to FPR look ok. However, I can't log it. Part of the time, it works ok, most of the time it does not which tells me its probably not the FPR though. Could the boost control solenoid be bad? Also a mitsu tech told me our cars don't like aftermarket O2 sensors, whcih i somewhat recently replaced with a Bosch. he suggested I buy the Mitsu one, but it is close to $200 bucks. thoughts?

 

[jshuman]

I dont know that this is an 02 sensor problem. I take it you have a logger. Can you log 02 volts to see if it is cycling porperly? Also do you have any boost leaks?

 

[2gGSX]

I run a Bosch O2 sensor without any problems, that's not your problem.

 

[TunaTalon]

Also a mitsu tech told me our cars don't like aftermarket O2 sensors, whcih i somewhat recently replaced with a Bosch. he suggested I buy the Mitsu one, but it is close to $200 bucks. thoughts?

At WOT the fuel/air ratio is running open loop. The O2 sensor has no effect on F/A ratio as long as you keep your right foot down. But don't fret, there is always somewhere else to spend those $$$$.

 

[Talesin]

Sounds like you're boost spiking and hitting fuel cut.

1) Boost leak test! Seriously. Build one and do it. Easy step by step instructions are up on VFAQ.
2) The Bosch O2 sensor is not your problem. I'm using one right now. Apparently there are O2 sensors available online for about $50, no need to shell out for an overpriced OEM unit.
3) When you put the new O2 sensor, how did you do it? Did you make sure that the O2 sensor is in tight and not cross-threaded? Did you make sure that when you were removing/installing, that you didn't shift the O2 housing loose of the exhaust housing on the turbo? An exhaust leak could cause boost spike.
4) Where is your wastegate boost reference coming from? Should be from the compressor outlet elbow. If you have a non-standard vacuum line setup (especially if it's teed off the BOV line) that could be causing a part of the issue.

 

[Flint]

Sounds like you're boost spiking and hitting fuel cut.

1) Boost leak test! Seriously. Build one and do it. Easy step by step instructions are up on VFAQ.
2) The Bosch O2 sensor is not your problem. I'm using one right now. Apparently there are O2 sensors available online for about $50, no need to shell out for an overpriced OEM unit.
3) When you put the new O2 sensor, how did you do it? Did you make sure that the O2 sensor is in tight and not cross-threaded? Did you make sure that when you were removing/installing, that you didn't shift the O2 housing loose of the exhaust housing on the turbo? An exhaust leak could cause boost spike.
4) Where is your wastegate boost reference coming from? Should be from the compressor outlet elbow. If you have a non-standard vacuum line setup (especially if it's teed off the BOV line) that could be causing a part of the issue.

O2 sensor is right, no cross-threading, housing is fine, I can check the wasitegate boost reference point. Where is the compressor outlet elbow exactly?

 

[Flint]

checked the O2 sensor function also. It shows the car cycling between lean and rich constantly, which I think is normal. The car goes rich at WOT up to about 10 PSI, stoich to about 13 psi then lean until i get a boost spike to about 17 psi then boost settles back to 14.5 psi. After about 12 PSI, the problem I am describing occurs. Until this happened, car was always, always rich at WOT

 

[2gGSX]

checked the O2 sensor function also. It shows the car cycling between lean and rich constantly, which I think is normal. The car goes rich at WOT up to about 10 PSI, stoich to about 13 psi then lean until i get a boost spike to about 17 psi then boost settles back to 14.5 psi. After about 12 PSI, the problem I am describing occurs. Until this happened, car was always, always rich at WOT

Do you have access to a logger, or are you just reading O2 voltages off of a narrowband gauge?

Assuming that what you're saying with the O2 voltages is actually occurring (I still don't trust those gauges), then what TunaTalon suggested earlier seems to be the culprit. I would suggest working your way up from the fuel pump itself to check for something that may have gotten clogged--the pump filter, the fuel filter itself, and finally, switch the FPR with one from another car that you know works.

 

[Flint]

I was using a mitsu scan tool to read the actual values. I will check fuel filter and fuel pump, but I am still thinking the boost spike which started at the same time, holds the key here. I just drove it and it is spiking to nearly 20PSI now and bucking like crazy. I have to figure this out before something breaks.

 

[Flint]

No more boost spike. I pulled the top vacuum line from where it plugs back into the intake and plugged the line. I now get about 13 psi of boost, no spiking. Does anyone know why this would be?

 

[skinnypimp]

have you tried fuel pump, sounds like a problem i had before. the car would shuttering, buck and the car wouldn't go over 20mph. i looked all over the car, i thought the intercooler piping came off or it had a leak somewhere. everything was fine then i changed the pump and the problem went away. that may not be the problem but it doesn't hurt to look in that direction.

 

[Talesin]

'Top' vacuum line? Not sure which one you're referring to. If you mean the stock BCS intake return line... well, theoretically the vacuum in the intake pipe would pull a little bit more of the boost reference away from the wastegate, letting it stay closed a second or two longer. If you're leaving that line dangling, you have a boost leak coming from that line, and should replace your BCS setup with a standard ball-and-spring MBC, straight line from the compressor outlet elbow to the MBC to the wastegate. No vacuum to contribute to your spiking problem, and no boost leak to rob power.

 

[Flint]

'Top' vacuum line? Not sure which one you're referring to. If you mean the stock BCS intake return line... well, theoretically the vacuum in the intake pipe would pull a little bit more of the boost reference away from the wastegate, letting it stay closed a second or two longer. If you're leaving that line dangling, you have a boost leak coming from that line, and should replace your BCS setup with a standard ball-and-spring MBC, straight line from the compressor outlet elbow to the MBC to the wastegate. No vacuum to contribute to your spiking problem, and no boost leak to rob power.

The top line of the stock BCS is now disconnected, not the lower one that goes to the wastegate actuator. I plugged the line and the spot in the intake it plugs into. Immediately pulled my boost level back in line, which I really don't understand.

 

[Talesin]

Yeah, that's the return line for the pressure the BCS bleeds off, so it isn't a boost leak, just returns to the intake tract. Now that it's capped off and that line is dangling, it's causing boost to leak out of that hanging line. Since that line is under vacuum during normal driving, and extreme vacuum under boost, it would create an artificial low, keeping the WG closed longer.. the boost would have to 'overcome' the latent vacuum in the line before it could trip the actuator. It's only supposed to bleed off a little bit, but with the BCS restrictor mod done, a lot more air can get through. Also, the BCS could theoretically partially fail... once you hit the fuel cut from overboosting, the ECU would close the BCS and bring the boost level back into stock trim, with no vacuum there.

Until you get an MBC, you could run a length of vacuum hose from the compressor outlet elbow to the WG actuator as a temporary measure. You'd run at 10psi (stock MHI WG actuator spring pressure rate), but wouldn't have that boost leak there.

I'd still STRONGLY suggest doing a boost leak test, in any case.

 

[Flint]

Thanks for the tips. I plugged the intake tract where the line went in and also plugged the return line from the BCS. It could be a failed BCS I guess. I am still having the miss-firing problem once i reach full boost but it is much better now that i have the boost under control where I am only hitting 13-14 PSI instead of the 18 PSI i was hitting before. I am going to go change plugs and wires and see if that cures the misfire i am getting. I guess the boost spike could be a seperate issue from the misfire but strange that it all happened at the same time.

 

[Flint]

If anyone can explain this, I would love to hear it. Just installed four new plugs. Misfire solved, overboosting solved. Everything is hooked up the original way, nothing is different other than the four new plugs. If i had a bad plug that was missing, would that cause a boost spike? I can't make this make sense.

 

[2gGSX]

Take it from an OCD kind of guy--it works now, so just let it go

 

[Talesin]

Well, the misfire is an easy explanation. Fouled or worn plugs, inconsistent gap, unreliable spark.
The spike could be caused by a misfire if it was bad enough... if the unburnt A/F mix was pushed out into the hot exhaust manifold and self-ignited there due to the ambient temperature... how antilag works (though it's sparked rather than self-igniting).

Haven't heard of it happening before, but that'd be my guess.