Hi,

Not exactly thirdgen, but I think most experienced TBI tuners read this forum so hopefully I could get some ideas

1992 S10 Blazer, 4.3l TBI. Symptoms - running rich in cells 30-60 kPa MAP/400-1200 RPMs, BLMs 108-113. Idle is slightly erratic and engine shakes somewhat - some say, it's normal for thes V6 engines. In other cells BLMs are pretty close to 128 and it runs fine.

Things I have done until now:
Measured fuel pressure - 11 Psi (seems normal to me)
Checked compression - around 160 Psi (seems OK)
Checked idle vaccuum - 22 in/Hg (seems fine to me too)
Changed all parts that would be considered as normal tune-up

Engine has pretty low mileage, someone had replaced it with new crate engine before I bought the truck. So I think mechanically it should be fine.

I tried to mess with VE tables and compensate sligthly those cells, but there was very little improvement - after changing VE about 10% the BLM number that was 108 before, was only 110-111 after. When I tried to change upper cells, only slight change in VE value changed BLM value like I calculated. It seems weird to me, I have no clue why it doesn't react as rapidly at idle and light load/slow RPM areas. Any ideas?

When I compare vacuum gauge reading and MAP reading, then MAP shows slightly lower vaccuum (if I calculated correctly). When the gauge reads 22 in/Hg, then MAP reads 37-38 kPa and it fluctuates slightly (maybe 1 kPa), but I think it's normal, as the idle is slightly rough too. Could that kind of difference mess fuel mapping so much?

I have no ideas what to do next, I should pass the emission test soon, but it would fail as HC reading is too high.

 

Check your base timing (with EST open) and measure actual timing at idle (if you have a dial back timing light it would make it easier).

//RF

 

I checked base timing and it was at 0 degrees.

I connected timing light to coil wire and aimed it to injectors and found some drips in one injectors spray pattern. Not much, but it seems that one of them leaks slightly. I powered the fuel pump manually and there were no drips anywhere, seems that dripping occurs only when injector pulses are occuring.

 

OK - then probably you may have a dirty injector - it is rather tough to say. The only way say with any degree of certainty is to swap offending injector with a recently serviced unit. Granted, it may be very difficult to do it at your location. I guess 4.3L TBI equipped cars (or trucks) are uncommon were you are.
I wish I could help, but it is only 5646 miles as the crow flies to Tallinn.

 

Well, I tried to swap injector pod with the one that was in my Firebird. And it got more weird.

As I understand, 5.0 TBI uses injectors with lower lbs rating - 40 lbs vs 45 lbs on 4.3. But AFR went richer - now it's rich on almost all areas, BLM ranging from 116 to 124. Cruising on highway 60 mph BLM pretty much stays stable at 122.

As almost all cells were rich and Firebird's fuel pressure was slightly high, I tried to play with BPW constant little bit. Original value was 181, I lowered it to 160 and there was practically no improvement in BLM range. Only time when BLMs are at 128 or so are high MAP and over 2000 rpms.

I checked the Firebird's injectors and they don't drip, so I have no ideas anymore...

 

No - that's not correct!! The 4.3L engines generally use 45 lb-Hr injectors. Depending on emission requirements there were some minor variation from that baseline fuel flow rate. The 5.0L TBI (LO3) injectors are rated at 55 lb-Hr. The baseline flow rate is performed at 13 PSI (just a hair under 1 Bar) fuel pressure. So the fact that your 4.3 engine went richer with 55 lb-Hr injectors makes perfect sense.

Cheat sheet for injector flow rates (@13 PSI)
4.3L -> 45 lb-Hr
5.0L -> 55 lb-Hr
5.7L -> 61 lb-Hr (LO5 truck)

//RF

 

Could anybody refer to some official flow data for different injectors - most internet sources refer 5.0 5235279 injectors as 40 lbs/hr.

At 55 lbs/hr rich fuel mixture makes sense , but I doesn't understand why changing base pulse width doesn't change considerably AFR?

I just checked my fuel pressure, it's stable 13 psi.

 

No no no, you DID make an improvement. You swapped to richer injectors but now EVERYTHING is richer. Before you only went fat at idle. And idle is where a slight injector leak can make a BIG difference. Idling involves only a TEEY TINY bit of fuel going into the engine. Doesn't take much of a leak to throw it way off. Above idle a slight injector leak is just a spit in the ocean- doesn't make much difference.

I think you just proved your old injectors were leaking and making it run fat (and unstable) at idle.

Make sense?

 

Yes, it makes perfect sense.
Actually I played with bigger injectors yesterday. Assumed that they flow 55 lbs/hr, not 40 and changed BPC to calculated 151.

BLM table went sligtly leaner, but the difference was smaller than I hoped. So I calculated VE corrections and changed VE main table accordingly. And that was improvement - AFR leaned out more than I hoped, so I had to compensate some cells slightly back.

Now most of the cells are inbetween 126 and 130, so that's not too bad, plenty of room for fine tuning, but close enough. Engine runs much better at low end, truck accelerates very good and doesn't shake at idle as much as before.

The only weird thing that I can't sort out - idle BLM stays contantly at 123. Correcting the main VE table changed it from 118 (where it was after writing in new BPC) to 123, but further corrections seem not to work anymore. My BPC is 151, so I guess Base Pulse Width isn't too short yet. There's no more tables that would deal with closed loop fueling than only main VE and VE adder tables, so I haven't found yet, what pushes AFR staying there.

Any ideas?

 

Possibly you have reached minimum pulse duration of TBI injectors (55 lb-Hr) at idle. TBI injectors have a mechanical limitation - they can only open, stay open and close so fast. So if commanded pulse width, at idle, is less than 1.0 to 1.1 mSec the injector will not stay open any less. This problem is very common when large injectors operating with increased fuel pressure at idle. I see this all the time with larger HP engines where you need more fuel at WOT (hence larger injector and more fuel pressure) and barely enough fuel at idle. For a stock like set up like yours larger injectors are not need - all you need is stock injectors that work well (no leaks).

//RF

 

What RF said.

Also, duoble check to make sure it's actually staying in closed loop at idle. They're known to sometimes drop out of closed loop at idle because the factory sensor just doesn't stay hot enough with so little exhaust gas going past it.

 

It's 4 wire O2 sensor, so it could not be an issue.