Not so long ago I posted about a small problem my car was having with it's BLM values, the problem is still here.

For a quick review here's the situation:

At some point in the not so distant past I was playing around with my data logging software and noticed something strange. The BLM values for the left side are in the red zone, 112 or so while the right side is in the green, 140 or so. Sometimes this split is more pronounced than others but it is always there. Typically the split is most noticeable at idle and at steady cruising. Heavier throttle tends to bring the value closer together.

I noticed the split sometime after I installed a set of 36# fuel injectors. Now before you guys rip my head off for running that injector let me explain. I'm looking for a set of 30's and I sold my 24's. In the mean time I have to run something. Are they too big? YES, I understand that. I NOTICED the split after I installed the 36's but that doesn't mean it was never there with my 24's. I didn't data log the car for the last month or more that I had the 24's and I didn't really data log it until well after I had the 36's.

What I can say is that my older data logs with the 24's show no sign of a split whatsoever. Somebody had mentioned that an aftermarket throttle body can lead to a BLM split. Although this sounds like a good theory I've been running my Holley 52 for a long time without any sign of a split and nothing with regard to it has changed. The only thing that I have knowingly changed has been the injectors.

With the injector change in mind I thought that the problem might actually be with one of them. I also had a set of 42's so I decided to give them a shot. I've now been running the 42's for probably 2 months and have the same BLM split. I feel pretty confident now that I don't have a problematic injector but the BLM split doesn't help me out from a tuning standpoint at all. In fact it would be compltely impossible to tune for driveability with such a split.

There is always the possibility of a vacuum leak but I really don't buy it. All my vacuum functions work just fine and my MAP pressures are right in line with that of other Hot Cam cars. Fuel pressure functions exactly the way it should and the car can easily provide enough fuel at WOT, I've dyno tuned that so I know it's not a fuel restriction.

There is only one other thing that I can think of that might be a problem, let me know how it sounds. The 92 doesn't have the fuel rail crossover at the front of the intake and it uses the batch fire fuel injection setup. Is it possible that what I am looking at is simply the fuel pressure differential between the two sides, amplified greatly by the oversized injectors? Is it possible that there is a small pressure difference between the banks equals a large fuel difference with the very large injectors?

Tonight I'm going to borrow a set of GM original stock 100% correct for this car 24# injectors. I'll put them in and see what happens. If it solves the problem I may just say to hell with it on the 30's. The 24's are taxed pretty hard at WOT but if they work from a driveability standpoint I may just stick with them. That would by far be the easiest way to get my MPG readouts to work correctly and the easiest way to tune for driveability. I still believe the 24's would be too small for extended WOT runs but I don't really make any extended runs.

Any thoughts?

 

Nathan, check out this tech tip. It may help.


hashmarks.com/techtips/throttle_body_mod.htm

 

Ive had this same problem on Block learn numbers my left is always low about 105.

right is dead on 128.

A call to ed wright confirms that its nothing in the programming. They suggested a leak somewhere (like leaking air (O2) before the oxygen sensor).

 

Hmmm, rt side blm 140=lean, lt side rich; lt side=drivers side....isn't the fuel feed on the passenger side??? So with that theory...then it should be the side away from the fuel feed side that would be lean.....

 

I'm at work, so I don't have the book in front of me; but, I think the Charles Prost book on Corvette Fuel Injection has an article about the batch-fire systems having a slightly different pulse width to overcome the difference in pressure from side to side.
That is the only place that I have ever read about a difference in the programming for right vs left side.

My '92, with the stock and slightly larger injectors does not seem to have any difference in the right vs left bank pulse widths or BLMs.

Tom Piper


[Modified by Tom Piper, 7:32 AM 8/13/2003]

 

Alright, I put the 24# injectors in it, of course it runs better part throttle, etc.

Now I'm down to a fairly reasonable BLM split at cruise 108 / 104 is fairly acceptable. It goes to an extreme of 118 / 104 sometimes but still fairly acceptable.

At idle it gets as bad as 130 / 100 but it goes back to normal as soon as I get off of idle. I'm thinking now that it might be the throttle body thing, I'll do some reading about that today.

It is nice having the MPG readouts working properly again though. Of course that depends on how you look at it. I was getting 30 mpg in stop and go with the 42's

:D

 

Check for any air/exhaust leaks btwn the MAF and the O2 sensors. If you don't have any, it's probably the throttle body. Have you tried putting the stock TB back on?

 

Well I don't have a MAF so I'm down to looking for leaks between the throttle body and the O2 sensors. Since the O2 sensors are direclty in the header collector I'm simplified even furter. My only places for leaks now are:

TB - Intake
Something on the TB
Something on Intake
Intake - Heads
Heads - Headers

I'm fairly certain there isn't a leak from TB to Intake, or anything on the TB. Intake is securely bolted down to the heads, can't imagine a leak there. That leaves the headers. Both O2 sensors seem to react just fine, I've got cooper gaskets and I can't hear any leaks. None of the bolts are loose so I don't see that either.

Not saying that it CAN'T be any of the above but I think for once I'm actually leak free.

I would try to put the stock throttle body back on it but I kinda gave it to a friend. I polished up a little bit, which meant I needed to take it apart, which means I now need gaskets for it. My friend has a 92 with a stock TB on it. I just need to buy a couple of TB to intake gaskets and just swap it out one day.

What I would like to do if I swap the TB back to stock is to head back to the dyno and see if it actually makes a difference with the full heads / cam package.

 

this might help http://para.noid.org/~lj/PCM%20Tutor...litBLMinfo.htm

:cheers:


[Modified by LT4POWR, 9:49 AM 8/13/2003]

 

That's a good article. That combined with the other one will probably fix my problem. I'll give it a shot real soon, maybe this weekend. At this point I'm pretty sure that is my problem.