hey guys this thing is driving me nuts. here the story, got a 91 sd system out of an f-body and put it in my 80 4x4 pick. the problem i am having is that when this thing goes into closed lood(verified with data master data log) it acts like it leans out. o2 volts drop to 0 and starts poping back through the intake, but this is the confusing part, the blm count goes to between 112 and 108. any insite into what is going on and just as importent how to corect it

 

Assuming that you have a heated O2 sensor it would appear that you have exhaust leak upstream from your NBO sensor or misfiring cylinder or two. Low O2 voltage indicates excess O2 in exhaust stream which ECM attempts to correct by reducing amount commanded fuel ('false' rich condition - low BLM).

If you have headers and a single wire O2 the problem stems from O2 sensor not being hot enough - you must run heated (3 or 4 wire) O2 sensor with headers. EOP

//RF

 

Do you still have air injection? That could be introducing air upstream from the O2 un-commanded by the ECM

 

no there is no air injection on it all the smog equtment has been removed from the engine and the prom. also there are no headers on it it has manifolds off a later modle chevy truck with the o2 sensor in the manifolds. i kind off have a hard time beliving that its an exuaghst leak. the whole system is new(except the manifolds) but no sounds like a leak and cant feel anything.

 

Is your timing set at 6*?

 

yes

 

I'm guessing you swapped your TPI setup onto the stock carburated engine? If so, that is your problem. I had a speed density setup on my old 80 K35 at one point using the stock carb engine and never did get it to run properly until I re-built the engine using EFI friendly parts. I chased my tail all over the place swapping injectors around, playing with the tune, heated O2 sensors, all the other sensors, fuel pumps and regulators etc etc etc, and it ultimately all boiled down to the mechanical components of the engine.

I wound up leaving the bottom end as it was since it was in very good shape and just swapped on a set of TBI swirlport heads. Lots of guys complain that they don't flow very well, but thats a moot point considering the injection setup's flow capabilities on a 350. Actually quite well matched. I replaced the camshaft with an eldelbrock TBI cam (the most mild EFI cam they had) and after that the engine ran pretty much as good as it would have if it came from GM that way. The thing pulled like a mule, and got really decent mileage in a truck with an NV4500 5 speed manual transmission, NP241, 4.10 gears and 35" super swamper boggers.

The problem is with the cam profile more than the heads. It lets too much raw air in through the valve overlap which confuses the poor ECM into thinking that the mixture is extremely lean despite actually being quite rich. It does not take much oxygen in the exhaust stream to mess things up. I went with the swirl port TBI heads becuase they are plentiful in good condition, already have the proper angle on the center intake bolts to match up with the speed density TPI intakes and are known for making solid down low torque like the TPI setup itself is known for. My engine ran out of breath by 4000 RPM, but that was plenty for what I used the truck for.

 

Assuming you have the check engine light wired in and you're not getting any codes...

You need to check all your grounds. Grab a multimeter and back probe the ECM connectors and check the resistance to ground for all of the black wires (including ones with white stripes). You shouldn't have any one wire with more than 2 ohms to ground.

There's also a tan wire at connector position E15. This is the O2 sensor ground. Make sure this wire sees less than 2 ohms to ground as well. The factory '91 O2 sensor does not have it's own signal ground and therefore the ECM relies on the assumption that you have a good chassis ground for the ECM and the E15 tan wire.

Here's a good list of the ECM pinouts....

http://chevythunder.com/fuel%20injec...ts.htm#1990-92 speed density f-body (ecm#1227730)

Scroll down to find the 90-92 list of circuits for 1990-92 speed density f-body (ecm#1227730) . What's good is that it'll tell you what voltage you should see on each wire during key-on and key-on-engine running. Swapping TPI's into non-EFI equipped vehicles can be tricky and frustrating. Just remember that the ECM has to be fooled into thinking that it's still in the original vehicle and that requires getting all of the input signals and grounds correct.