1995 R33 Gtr - Twin Bovs Going Off In Sequence And Different Readings On The Two Boost Gauges

[judasentinel]

I have a 1995 R33 GTR, and recently, I have seen it develop an issue relating to the turbos. This is what has been happening:

Light throttle, the car shudders and jerks a little bit, but when I accelerate hard, the jerking goes away. This only happens around 2000-3500 rpm range, and only under light throttle. If I let go of the accelerator, the shuddering goes away.

I have two boost gauges, possibly one for each of the turbos. One shows a slightly higher reading than the other, which makes me wonder why it is so. When I bought the car a few months ago, both gauges used to show almost the same reading.

The two BOVs seem to go off in sequence. One goes off first and then a second later, the other goes off. This wasnt the case a couple of months back.

I am stumped as to what the cause is, how to look for it, and what to check. Could someone please help?

Thanks

Munib

[niZmO_Man]

Tried another pair of AFMs?

[judasentinel]

Never occurred to me that the AFMs could cause the readings on the boost gauges to be different or the BOVs to go off one after the other. Do you think that could be an issue? When it is idling and I give it gas, one boost gauge jumps to 0 and the other goes a little futher up. And when I turn the car off, the boost gauge that goes to 0 goes back to 0, while thge other one stays much lower. Arent they both supposed to go back to zero pressure (no vacuum or boost) when the car is off?

Could this be caused by a leak or faulty coil(s)?

How is what I am describing (the shudder and jerking) different from turbo shuffle? Or is it?

[slowridn73]

It's your AFMs. Possibly a loose afm plug but most likely it needs to be replaced.

Boost gauges go out of calibration, but they should be reading off the same system. What I mean is that you have two turbos but they feed into a single intake system. I assume there were two installed to read the pressure drop across the inter cooler... maybe?

[judasentinel]

I wasa thinking that the persopn who I bought the car from in Japan had two boost gauges to measure the boost pressure before the intercooler, which hardly makes any sense. So I wonder why two gauges in the first place? Assuming your point about one auge measuring pressure before and the other after, the intercooler, is valid, how does it correspond to the shuddering, in that the difference between the two gauges is much higher than it was, say, a week ago?

I am wondering if it has ANYTHING to do with the possibility of onw of the turbos not getting enough exhaust pressure, so it is spinning slower, and therefore, producing less pressure. If that were the case, it would point to one or more cylinders not firing properly.

Or is the above totally wrong? I would rather it be a dirty MAF, than a faulty set of coils or a timing belt issue (skipped a tooth or two). The idle is also slightly rougher than a few days ago. Could this be related to poor combustion/ignition in a cylnder?

I appreciate any and all inputs.

[r32-25t]

If both boost gauges don't both go back to the same reading when the car is switched off then one is faulty

After reading your posts I strongly suggest taking the car to a professional

[slowridn73]

You skipped the first part of my post. I had similar issues and it was the MAFs. Especially the shuttering at 2000-3500, it's a limp mode that in that rpm range is specific to MAF failure.

Forget about your boost gauges for right now and take care of you MAF issue.

[judasentinel]

@brett: I understand that they both should go back to the same level when the car switched off....and I fixed it, yay!!

However, I don't feel my ignorance in the matter requires me to take the car to a pro at this stage. Yes, if I am unable to identify the real cause and cannot fix it, will I do that.

@aaron: The limp mode generally doesn't allow the car to rev past 3000 rpm or thereabouts. Mine revs just fine. The slight jerking only happens in gears 3-5 in that rpm range and once I am past the 3000-3500 rpm range, it runs great. In addition, the car doesn't do it in that rpm range in second or first gears, possibly because there is a higher level of load. I am feeling that this might have something to do with the load levels or the TPS settings? I have had many R32s and never had to deal with this issue with them. Wondering if there is something different in the R33s. I admit that since this car was already modified, and I am still discovering the extent of the mods, I might not know everything about her.

I just drove her to work today, and other than the jerking around 2500-3500 rpm in gears 3-5, it was great. All 6 cylinders seem to be firing, and the car is building boost well. I will try to clean the MAFs soon and see what change that causes.

Thank you both for your inputs. Please do keep the suggestions coming in.

Munib

[slowridn73]

Ok, I miss understood the symptoms. Just a heads up though, as my mafs were going out they had very similar symptoms although it was apparent in all the gears, not only in 3-5. Usually the car would jerk a few times then accelerate. Usually this occurred after I was stopped and seemed to be a result of heat soak? In addition to cleaning you may want to have the mafs tested and check the ground on plugs.

[judasentinel]

A lot of black soot comes out the exhaust when I start the car in the mornings. It seems she is running a bit richer than normal, so that might be causing the plugs to foul up, especially in the rear 3 cylinders, and subsequently, the rear turbo might not be getting a stronger exhaust flow. I intend to try out the coils when it gets a little warmer, to see if the spark is strong or weak in all of them.

Other than checking the coils and plugs, along with the ignitor, and the MAFs, what else should I check? Is there a write-up on how to check the TPS voltage properly on the car?

Could a boost leak also cause this kind of jerking, which goes way when there is enough velocity in the charge pipe that prevents the air from escaping?

I have already ruled out the fuel pump and CAS as the causes, since they would make the car undriveable under any level of load.

[niZmO_Man]

Go to the tutorials section and search for AFM solder fix.

[judasentinel]

The MAFs are fine. I checked and cleaned them up. The jerking continues, and seems to happen only under partial throttle, when the car has warmed up. I am thinking coils/coil harness or TPS. The plugs are brand new.

Also, the turbos spool up as soon as I press the throttle, and upon letting go, I hear the swoosh swoosh of the BOVs, but I hear two sounds - one after the other. That doesn't seem normal to me. My R32s never had two BOV sounds. What does that mean?

Thanks for the inputs.

[niZmO_Man]

Did you do the solder fix? You can find a pair of used ones for $50.

[slowridn73]

I soldered mine and refreshed the grounds. I still ended up replacing them to fix my issues.

[niZmO_Man]

I'm on my second set (R33 GT-R ones). Original solder fixed AFMs lasted about a year. Current ones are okay but not 100% with them.

I should fix my Nistune up and throw the aftermarket AFMs on there.

[r32-25t]

I've got 2 set of AFMs sitting around and both have an issue so the ecu is going in the bin and being replaced with a haltech

[judasentinel]

How wrong is it for me to assume that since the jerking happens only when the car is running hot, that it relates to coils and not the MAFs?

[niZmO_Man]

Not wrong at all.

Although my car did something similar, AFM solder reduced it greatly. New coils+spark plugs pretty much fixed it.