Most blm's to 108 - next step advice please

[piperclipper]

1991 GMC k2500 pickup 350 TBI all stock. 236,000 miles. Most blm's head down to 108, but tailpipe doesn't look or smell rich. Runs good only at map's above about 80 - highway uphill for example. Otherwise feels like a miss. Although the ecm thinks it's rich, also recording lot's of knocks, but don't ever hear a ping.
1. Found/Fixed vacuum leak at tbi base.
2. Fuel pressure fine @ 13 at filter.
3. New ignition parts. (ignition module, cap, rotor, wires, plugs).
4. Presently trying premium no alcohol fuel with techron injector cleaner. I might guess dripping injectors if techron makes no difference. (Fuel sprays look like a cone in driveway)

I'm logging on tunerprort assuming $42-1227747

Any helpers on suggested troubleshooting next steps?

[steveo]

low BLMs mean it is removing fuel. it doesn't really mean the ECM thinks its rich, it means 'i think i WAS rich but i went and fixed that'.

in this case it has made a large (~18%) correction. but, if this assessment is false (o2 issue?) that means you are actually running lean.

108 is the limit too, so it's gone as far as it can

since it's bone stock this is abnormal.

besides a badly leaking injector or severe sensor fault it's really really hard to make a fuel injected engine run rich. since your engine is an air pump, usually an ignition or mechanical issue pumps oxygen into the exhaust which reads as lean drives your trims UP (adding fuel).

i haven't looked at your log but make sure your TPS,MAP, and coolant temp readings are sane. after that, how about purposefully fault the o2 by unplugging it and see if it runs better?

[Fast355]

low BLMs mean it is removing fuel. it doesn't really mean the ECM thinks its rich, it means 'i think i WAS rich but i went and fixed that'.

in this case it has made a large (~18%) correction. but, if this assessment is false (o2 issue?) that means you are actually running lean.

108 is the limit too, so it's gone as far as it can

since it's bone stock this is abnormal.

besides a badly leaking injector or severe sensor fault it's really really hard to make a fuel injected engine run rich. since your engine is an air pump, usually an ignition or mechanical issue pumps oxygen into the exhaust which reads as lean drives your trims UP (adding fuel).

i haven't looked at your log but make sure your TPS,MAP, and coolant temp readings are sane. after that, how about purposefully fault the o2 by unplugging it and see if it runs better?

Not hard to make it run rich if the MAP or CTS readings are wrong because the sensor has gone bad. I have seen many examples of both fail over the years on TBIs.

[Fast355]

Another TBI specific issue I have run into is the air cleaner thermal device. It controls a flap in the air cleaner that either pulls heated air off the exhaust manifold from under the heat shield or pulls cool air from through the fender. The control valve modulates the flow of air from both places to delivery a fairly steady temperature to the engine. Pull the tube off going to the fender. With a cold engine the plate should be closed to the outside air source. Start the engine an let it warm up, as the engine warms up the plate should partially open at a minimum. I have seen something as simple as a missing hot air supply tube cause the engine to run poorly in the stock configuration.

[piperclipper]

Previously found the air cleaner thermal flap not opening when warm. I just have it blocked open. Are parts available to fix? Did a quick search, but couldn't find anything.

[piperclipper]

low BLMs mean it is removing fuel. it doesn't really mean the ECM thinks its rich, it means 'i think i WAS rich but i went and fixed that'.

in this case it has made a large (~18%) correction. but, if this assessment is false (o2 issue?) that means you are actually running lean.

108 is the limit too, so it's gone as far as it can

since it's bone stock this is abnormal.

besides a badly leaking injector or severe sensor fault it's really really hard to make a fuel injected engine run rich. since your engine is an air pump, usually an ignition or mechanical issue pumps oxygen into the exhaust which reads as lean drives your trims UP (adding fuel).

i haven't looked at your log but make sure your TPS,MAP, and coolant temp readings are sane. after that, how about purposefully fault the o2 by unplugging it and see if it runs better?

1. Coolant temp is a little low - running 185-190 F. Is that low enough to switch to a higher rate fueling table? How would I look that up? TPS & MAP look good.
2. Tried test run with O2 disconnected as suggested. Runs noticeably worse, rich smelling at idle.
3. Tried test run with map electrical disconnected. Had high hopes since I thought the low map reading would use a smaller fuel table and keep blm's from bottoming out, BUT the low map error code must prevent blm's from changing. Int just went as low as it could go.
4. Let me know if you think I learned anything. Runs great with map above 80, int rises to 124 or above and blm's rise above 108. Is an 80 map a breakpont of some kind?

[piperclipper]

I am surprised at how little correspondence there is between TPS voltage and MAP. In other words at small amounts of throttle MAP might be anywhere from 30 to 80. Seems like a high map at low throttle would ask for too much fuel. Anybody know how to compare my TPS vs MAP with a known good running 350?

[tayto]

get the injectors professionally cleaned or buy new injectors

[piperclipper]

Sent to Witch Hunter for clean & test. Still runs the same. Anymore ideas?

[In-Tech]

Hiya,
Sounds to me like EGR is entering the manifold. Either at the valve or the crossover is cracked, I've seen that a few times.

[piperclipper]

Hiya,
Sounds to me like EGR is entering the manifold. Either at the valve or the crossover is cracked, I've seen that a few times.

I used a hand vacuum pump to test. Diaphragm moves and vacuum holds. Is the ecm signal that opens the solenoid available in tunerproRT $42-1227747-V5.2.adx?

[piperclipper]

Maybe this is a big hint to someone - Runs really bad at transition from stopped at idle to traveling down the road. Closely monitoring Map kPA and TPS, I see that Map kPA jumps up dramatically as small amounts of throttle is added. 10-15% tps results in Map kPA jumping from 30 to 80 and beyond. Sometimes it hits 100 and sets the 33 map high code. Once on the highway, it runs good. Stop & go in traffic it runs really bad.

[piperclipper]

Maybe this is a big hint to someone - Runs really bad at transition from stopped at idle to traveling down the road. Closely monitoring Map kPA and TPS, I see that Map kPA jumps up dramatically as small amounts of throttle is added. 10-15% tps results in Map kPA jumping from 30 to 80 and beyond. Sometimes it hits 100 and sets the 33 map high code. Once on the highway, it runs good. Stop & go in traffic it runs really bad.

Testing EGR & solenoid with vacuum gage & pump and they both seem to check-out. However, if I leave the egr valve vacuum line disconnected, it runs better, and it seems like the big jump in map at just a little throttle goes away. My guess is ECM is signalling solenoid to open EGR valve too often. Anyway to check that out? Can anyone look-up exactly what inputs cause the solenoid to open? Manual only says the following, but it's not much help troubleshooting.
"EGR SYSTEM CHECK
2.5L (S)# 4.3L (M/L, C/K, G), 5.0L (C/K), 5.7L (C/K, R/V, G) EXCEPT VEHICLES WITH
4L80-E TRANSMISSION
Circuit Description:
The ECM operates a solenoid to control the exhaust gas recirculation (EGR) valve. This solenoid is normally closed. By providing a ground path, the ECM energizes the solenoid which then allows vacuum to pass to the EGR valve. The ECM control of the EGR is based on the following inputs:
• Engine coolant temperature - above 25°C.
• TPS-"OFF” idle
• MAP

[MO LS Noobie]

One way to test the EGR is to see if it has internal leakage. Disconnect the vacuum line and plug it. Then start the cold engine. The EGR valve should be closed now and if you feel around the base of it it should stay the temperature of the manifold. If it gets significantly hotter within a minute or two it is leaking and needs to be replaced. You can try cleaning it by removing it and seeing if carbon is stuck in the pintle/seat area. If that checks out okay, clear all of your adaptive values and go for a test drive with the EGR valve still plugged.

At this point, if it runs well, either the exhaust back pressure is excessive (if it is a BPEGR, back pressure EGR), the EGR valve itself is defective and opening to quickly, or the small solenoid valve that controls the vacuum to the EGR is defective (quite common), the only other consideration is a wiring problem or the PCM itself.

Hope this helps, Jeff

[KeepItRunnin]

I agree with MO LS Noobie, all good things to check / try.

Also, does EGR valve get vacuum from a port on the TB? If so, maybe the return spring in the EGR valve is weak, allowing it to open too much too soon? Or for some other reason it's getting too much vacuum too soon? Which would be fairly easy to monitor while driving with a "T" and some hose and gauge, but you'd need some idea of how much vacuum and when- a profile of vacuum vs. throttle position (if there is a port, iirc there is).

[piperclipper]

I agree with MO LS Noobie, all good things to check / try.

Also, does EGR valve get vacuum from a port on the TB? If so, maybe the return spring in the EGR valve is weak, allowing it to open too much too soon? Or for some other reason it's getting too much vacuum too soon? Which would be fairly easy to monitor while driving with a "T" and some hose and gauge, but you'd need some idea of how much vacuum and when- a profile of vacuum vs. throttle position (if there is a port, iirc there is).

Closing this out so maybe helps the next person. It was a weak spring in the EGR valve so that it opened too much/soon. The way the GM manual tests are written it still passed all tests. Replaced with Delco 214-5073 from rockauto. Now runs great.Thanks!

[KeepItRunnin]

Wow, I guessed right for once. Now if I could just do that for my own cars... Glad I could help and thanks for letting me know.