Knock only under boost...

[90 GSX]

I have searched already... the only answer I could get that applies to my situation is that hot spots from running rich over time maybe be causing the knock under boost. The answer was to seafoam but I seafoamed it once to no avail... I thought this was the problem because I had my ECT sensor wire incorrectly so it was running very rich for a time until I fixed it. The motor is fairly new though and the seafoam didn't do much.

The only things I can think that would make a difference at this point are as follows:

-I'm missing the gasket between the o2 housing and downpipe. I didn't think a gasket was needed there because the dp has a lip that fits pretty snugly into the o2 housing...
-The spark plugs are still around from the time it was running rich. I will replace them tomorrow.
-I took the aircan off but I created a makeshift barrier from aluminum insulation. It's not airtight though but I don't hear about people having problems with removing the can. At least not knock.

What else could it be? I'm getting like 20 or 25 counts even sometimes, ONLY when I am under boost. I could drive around all day long without boost and not a single peep out of the knock sensor. The second I hit 1 or 2 psi I get a 5 or 6 counts. Then at 11 psi I am getting a whole bunch of knock.

It sucks because I feel the car pull really nice for a split second before the timing gets beat down to a little pulp (ie 10-12*).

Help! Oh and I get a random miss at idle too... probably due to the aircan removal but I don't know.

 

[andymoraitis]

first things first:

1. Check your base timing and verify that it's at 5 degrees BTDC
2. Get a gasket in beytween the turbo and O2. Boost leaks before the O2 are disastrous to performance.
3. Verify that the knock sensor doesn't have any kind of goo around the back of it. If so, replace it.
4. Check voltage to the fuel pump and make sure it's not petering out under load.
5. Has the fuel filter been changed recently? If not, please change it.

Give those a shot and then let us know what you find.

Andy

 

[90 GSX]

Thanks for the reply.

1) Base timing is at precisely 5*. I have checked it a zillion times... One thing that is peculiar is that when the car has that little miss at idle I was talking about, the timing gun does not flash.

2) The gasket that's missing is the exhaust gasket between the downpipe and the o2 housing.... it's near the o2, but after it nonetheless. I will fix this though when I put new plugs in. Where can I get a downpipe to o2 housing gasket other than the dealership?

3) Knock sensor was replaced with the motor with a brand new unit from mitsu. I used a N/T block for the rebuild but I am sure that it is in the right hole. Also, when I put it in, I spun the sensor down by hand to zero lash, then got a wrench on it and barely put pressure until I felt the sensor torque ever so slightly. I thought that was good, but do you think it could be too much torque? Would it cause knock only under boost?

4) How can I check the voltage while driving? I have a datalogger but it only gives me battery volts which is usually around 13.5 give or take depending on the rpm. The fuel pump is rewired with a bosch relay and 8 gauge wire directly from the battery to the connector inside the panel in the trunk.

5) Fuel filter is a brand new mitsu unit. Unfortunately, the old one was stuck on there so hard I bent the line while removing it. I just got some fuel line cut and sanded the bent part out and reconnected it. Never had a problem with it... BTW the car is running RC 550's with an eprom chip from dsmchips adjusted for them.

 

[Quasimondo]

What about your fuel pressure regulator? If that's gone south, it may not be raising fuel pressure like it's supposed to when your turbo spools up. I had a similar problem and figured it out by clamping down a bit on the return line with a vise grip (I zip tied it as well). That should raise the fuel pressure, and if the fpr is bad, the knock count should be reduced since the fuel pressure has been raised.

 

[yokotabrat]

Have you tried race gas or higher octane gas? It seems like real knock, but it's always a good test with high octane fuel to see if there is any knock reduction.

Does your TMO chip have the injector compensation updated for the 550s listed in your profile? Was MAS are you using? Are all the honeycombs in place? Have you logged the MAS hz output, air temp and baro to make sure they are doing what they are supposed to be doing?

 

[90 GSX]

I swapped the fpr out with one from a car that I know works and it did nothing...

I have not tried race gas. I was going to at the track when I went last month but the race gas guy left early for the day. Can I try xylene or toluene without risking the o2 or any other part getting damaged? I don't know if I really want to use acteone, it's a pretty powerful solvent.

The chip is compensated for the 550's. It's a 1g MAF, all stock, no modifications to it except removal of the aircan. I cut the lip where those three clips seal and one bolt hole so it would be sturdy.

 

[90 GSX]

Just changed the plugs. Old ones are white as a ghost.

 

[yokotabrat]

I don't have much experience with xylene or toluene, so I'm not much help there. I've used NOS brand racing forumula octane booster before and noticed a slight improvement in knock reduction. I only noticed an improvement with the racing forumla (with nitromethane supposedly), though.

Do you have a logger? Have you looked at fuel trims(all three). They might point in the right direction to see if you are running rich or lean.

 

[90 GSX]

My fuel trims fluctuate a lot around 100%. Say for example if I stopped and my radiator fans came on. My trim would start going up as will my IATs because of the missing can. I can get exact numbers and post them tonight.

I also have a slight hesitation that disappears when warmed up. So I'll be cruising and feel a surge back and forth when I am applying constant throttle. I think it might be that exhaust leak I was talking about. Any comments on taht?

 

[tom04841]

That exhaust leak is what is causing it to run real rich. Air is geting thru where the gasket is supposed to be and making the 02 think you are running lean, so it is dumping fuel to compensate.....i had the same problem. Put a gasket in and took care of it.

 

[90 GSX]

Okay so does anybody know where I can buy a downpipe to o2 housing gasket without paying 20 dollars + shipping on ebay??? Do I need to go to satan??