High BLM numbers at speed
Thread Starter
Intermediate
Joined: Mar 2005
Posts: 41
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High BLM numbers at speed
'95 LT1 with 140k (stock)
I've narrowed down my bad fuel economy and found that the BLM numbers are getting high when on the interstate. Datamaster histogram shows at engine speeds above 2000 Rpms.
LTerm- about 138-142
STerm- 124-130
Around town and at idle the numbers are great.
The fuel filter has been changed and fuel pressure was checked driving at high engine speed. The air pump is off.
One theory I have is that a Cat is clogged forcing too much EGR gas into the intake causing it be too lean. Backpressure on one side was checked and found to be ok (less than 2 psi at 2000 Rpm).
Another guess is clogged injectors. Can anyone help?
I've narrowed down my bad fuel economy and found that the BLM numbers are getting high when on the interstate. Datamaster histogram shows at engine speeds above 2000 Rpms.
LTerm- about 138-142
STerm- 124-130
Around town and at idle the numbers are great.
The fuel filter has been changed and fuel pressure was checked driving at high engine speed. The air pump is off.
One theory I have is that a Cat is clogged forcing too much EGR gas into the intake causing it be too lean. Backpressure on one side was checked and found to be ok (less than 2 psi at 2000 Rpm).
Another guess is clogged injectors. Can anyone help?
Last edited by larrynash; Mar 27, 2006 at 04:03 PM.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
2psi shows that the exhaust is getting out - Assuming that it's not isolated to one side, I'd suspect the injectors and you might want to try a balance test. Otherwise, assuming the O2's haven't been replaced in 140k, I'd try new ones and recapture. I'd also check compression, get a vacuum reading.
Thread Starter
Intermediate
Joined: Mar 2005
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MAF question
Thank you for the input-
O2 sensors were changed last year. Vaccum at idle is 19.5 inches Hg. Could a MAF sensor be bad only at higher flow rates?
O2 sensors were changed last year. Vaccum at idle is 19.5 inches Hg. Could a MAF sensor be bad only at higher flow rates?
Last edited by larrynash; Mar 29, 2006 at 07:48 PM.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
I don't think so as my experience with MAFs are that they read too little or too much regardless of cell. Assuming you're valve train is good and that nothing is contaminating the mix (oil or water which would again probably be evident regardless of throttle), I'd suspect that the injectors are restricted. A balance test would probably confirm or rule them out. That can be tricky, because you're looking for small differences between injectors and most gages have the spread so narrow that you can't see it. You'll also need to energize each injector. NAPA sells a tool, but it will set you back 80 Bucks. Might be better to see what a dealer will clean them for.
Thread Starter
Intermediate
Joined: Mar 2005
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Charcoal Canister
I feel like I've been all over the map with this problem. Today I looked at the datamaster file further. At speed, the high fuel trim values are coming up when the EGR and Charcoal canister solenoid opens. The MAF signal looks like it matches the engine speed well. For some reason when I disconnect the MAF all is well. Does the EGR and CC stop operating when the MAF is disconnected? I'll check the lines to the CC next.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Once a code is set - EGR, CC, Torque Converter Clutch and couple of other things are disabled.
If you want to troubleshoot the CC, simply crimp off the hose. The ECM has no idea if anything actually passes through it.
If you want to troubleshoot the CC, simply crimp off the hose. The ECM has no idea if anything actually passes through it.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
It occurred to me - and maybe it allready has to you too - that if the canister was plugged up or the valve was bad, then it could very well be sucking air when purge was commanded. I do think that if there was simply a hole in the hose, it would lean out in all the cells (and you'd probably smell the gas fumes). Anyway crimping off the hose with some vise grips or putting a vacuum cap over the TB inlet would probably confirm or rule it out.
Thread Starter
Intermediate
Joined: Mar 2005
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Crankcase Ventilation
The carbon canister system turned out to be OK. The hoses were cracked but not leaking. The valve worked fine. The normal operation of this system is causing the problem to be at its worst at speed.
I did find the fresh air tube for the crankcase ventilation system was cracked by the throttle body. I checked the PCV system according to the manual by blocking the inlet and putting a vacuum gage on the dipstick tube. Pressure was building up in the crankcase. I cleaned out the vacuum side and reduced pressure to zero. I may pull the intake next to clean further. Why would this cause high BLM numbers?
I did find the fresh air tube for the crankcase ventilation system was cracked by the throttle body. I checked the PCV system according to the manual by blocking the inlet and putting a vacuum gage on the dipstick tube. Pressure was building up in the crankcase. I cleaned out the vacuum side and reduced pressure to zero. I may pull the intake next to clean further. Why would this cause high BLM numbers?
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Burning Brakes
Joined: Apr 1999
Posts: 758
Likes: 1
From: Cumming GA
Originally Posted by larrynash
'95 LT1 with 140k (stock)
I've narrowed down my bad fuel economy and found that the BLM numbers are getting high when on the interstate. Datamaster histogram shows at engine speeds above 2000 Rpms.
LTerm- about 138-142
STerm- 124-130
Around town and at idle the numbers are great.
The fuel filter has been changed and fuel pressure was checked driving at high engine speed. The air pump is off.
One theory I have is that a Cat is clogged forcing too much EGR gas into the intake causing it be too lean. Backpressure on one side was checked and found to be ok (less than 2 psi at 2000 Rpm).
Another guess is clogged injectors. Can anyone help?
I've narrowed down my bad fuel economy and found that the BLM numbers are getting high when on the interstate. Datamaster histogram shows at engine speeds above 2000 Rpms.
LTerm- about 138-142
STerm- 124-130
Around town and at idle the numbers are great.
The fuel filter has been changed and fuel pressure was checked driving at high engine speed. The air pump is off.
One theory I have is that a Cat is clogged forcing too much EGR gas into the intake causing it be too lean. Backpressure on one side was checked and found to be ok (less than 2 psi at 2000 Rpm).
Another guess is clogged injectors. Can anyone help?
I've also seen worse BLMs and this will not contribute to poor milage. The BLM is triming to statisfy the O2 and the A/F is corrected unless the BLM hits the shelf and there is no more trim left. Its doing what its suppose to do. I could see poor economy comming from plugs, wires or fuel filter or just low compression thus more blowby and less effeciency.
John
Thread Starter
Intermediate
Joined: Mar 2005
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Compression
I hope this is now fixed, but I think I will check compression. It should be ok. I have no oil burning, good oil pressure (20 psi) and a very clean engine without any strange noises.
What do you mean by related to cylinder heads?
What do you mean by related to cylinder heads?
Last edited by larrynash; Apr 2, 2006 at 09:25 PM.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
If it was the PCV or fresh air hose and all is normal now, sounds like it's fixed. A cracked hose is going to suck in air which is just like a vacuum leak. It's unmeasured by the MAF, but the O2 picks up the lean condition and adds more fuel. Usually, if its stays around 142 to 144, it'll set a code. Checking compression, given the mileage, isn't a bad idea, but it sounds like everything is in good shape.
Intermediate
Joined: Jul 2000
Posts: 26
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From: GLEN ELLYN, IL, USA
Any luck with this problem???? My 96 is doing the same thing....BLMs pegged @ 160 at mid/high throttle, ok on light throttle and idle. Fuel pressure is good. Once the MAF is disconnected, the BLMs come back down and the car runs fine. Found a small leak on the PCV elbow and fixed that, but it did not improve anything. I get P1173 and P0100 I believe which are 'running lean, trim maxed out' and MAF performance.
Thanks
Tom
Thanks
Tom
Thread Starter
Intermediate
Joined: Mar 2005
Posts: 41
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Problem Solved
Yes- My PCV system on the vacuum intake side was clogged. Normally air should be sucked from the fresh air intake tube. Instead, cranckcase gas was being injected into the intake, bypassing the mass air flow sensor.