biker
Poster
Posts: 55
Reg: 12-25-23
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10-15-24 05:59 AM - Post#2874713
I posted this in the 88-98 page, but thought I may get a few more eyes on it here.
91 k2500, 8 lug axles, SM465 manual trans, 5.7.
-Fuel pressure is steady at 13 psi.
-great starting, idle and driveability through entire rpm range when cold
-New distributor, cts, knock, heated 02, map,egr valve and egr module.
Once up to operating temp, I get a stumble under load. So, 2nd and third gear, trying to accelerate, it will hitch and stumble until load reduces. High rpm is fine, and it will easily pull a hill at hwy speeds as long as throttle input isnt too heavy.
It is a 5.7 with a factory roller cam that the block stamping says came out of a caprice.
The only thing I have not done is clean or replace the injectors which appear original. Very hard to find new, high quality injectors.
My thought is that when in open loop, efi is running a bit rich until warm when fuel is managed by the sensors and ecm. So, the injectors seem to have the capacity to supply enough fuel, but something from a sensor is telling the ecm to stay lean under load.
TPS and IAC are also still original. Another odd behaviour is when coasting off throttle, there is a steady, rhythmic engine-braking effect that you can feel and hear through the exhaust. Its like the injectors are on/off every 2 seconds while coasting.
Any help appreciated!!
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 Shepherd
Valued Contributor
Posts: 3603
Loc: Lake George, NY
Reg: 11-11-15
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10-15-24 06:29 AM - Post#2874714
In response to biker
How did this start to happen.If you could scan it, you could see if the o2 sensor once in closed loop is driving the mixture to the lean side, causing the condition.I thought 91 trucks did not have a heated o2?
Edited by Shepherd on 10-15-24 06:40 AM. Reason for edit: No reason given.
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biker
Poster
Posts: 55
Reg: 12-25-23
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10-15-24 02:45 PM - Post#2874727
In response to Shepherd
Thank you! It started after top end gaskets, new distributor etc. I have no performance baseline because I bought it non-running.
I do have a decent OBD2 scanner that does OBD1 as well. I just dont know what parameters I can expect to see.
I will see what it says tomorrow.
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biker
Poster
Posts: 55
Reg: 12-25-23
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10-15-24 02:50 PM - Post#2874728
In response to Shepherd
It did not come with a heated 02 sensor, but I bought one and an adapter to plug it into the factory harness. It has a ground and 12v wire which are all wired properly.
I used the heated 02 because I removed the original giant cat convertor thinking it may have been plugged. My thinking is the heated 02 sensor will be more efficient with the lowered exhaust backpressure.
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Shepherd
Valued Contributor
Posts: 3603
Loc: Lake George, NY
Reg: 11-11-15
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10-15-24 04:46 PM - Post#2874730
In response to biker
I think I would replace it with the appropriate one for that year engine and pcm, it maybe calibrated incorrectly for your combo.
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biker
Poster
Posts: 55
Reg: 12-25-23
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10-16-24 04:09 AM - Post#2874734
In response to Shepherd
Ok, will do. I will try my scanner first, and see if it will give me anything. Would be nice to get live data, here's hoping.
I was hoping I would get a dtc if the O2 sensor was an issue. Sigh....
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biker
Poster
Posts: 55
Reg: 12-25-23
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10-17-24 06:04 PM - Post#2874794
In response to biker
Well, my scanner was able to tell me the historical numerical codes, even after I disconnected the battery for the afternoon. I thought they would be erased? I haven't had a service engine light come on once since I fixed the previous issues, but this ecm has a memory like an angry wife.
Anyways, I could not get it to display live data.
Is there a program you guys like to use for laptop to display live data?
Thanks again.
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biker
Poster
Posts: 55
Reg: 12-25-23
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10-20-24 08:00 AM - Post#2874867
In response to biker
Just ordered an OBD1 to USB cable for my laptop. Downloading Tunerpro RT today. Interested to see what's going on with timing, 02, etc.
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Shepherd
Valued Contributor
Posts: 3603
Loc: Lake George, NY
Reg: 11-11-15
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10-20-24 04:48 PM - Post#2874872
In response to biker
Keep us in the loop, especially note your o2 readings.
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biker
Poster
Posts: 55
Reg: 12-25-23
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10-21-24 03:56 AM - Post#2874875
In response to Shepherd
Yessir, will do.
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biker
Poster
Posts: 55
Reg: 12-25-23
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10-27-24 08:07 PM - Post#2875087
In response to biker
Update:
Well, never assume your own work cant be substandard....
While waiting for my OBD1 to usb cable, I figured I would go through some basics. Ohm'd out the cap, wires and plugs, inspected cap, rotor, coil, wiring etc.
Stuck the timing light on and discovered base timing was 6 degrees retarded...
Went thru my notes made when I first assembled it, and I wrote specifically "timed while cold". I was so excited that it ran right off the bat that I timed it right away, slammed the hood and went for a drive. Never re-checked once the engine was warm. So today, I set base timing to 0, hooked up the ecm wire and it runs great. I'd say 80% better. I can still make it stumble if really try on purpose,but it has more power now and pulls great under load.
Looking forward to looking at some live data to dig in a bit deeper.
Also noticed that the air cleaner snorkel heat diverter valve is staying closed. I propped it open for now just tule out air flow as a problem.
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Shepherd
Valued Contributor
Posts: 3603
Loc: Lake George, NY
Reg: 11-11-15
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10-28-24 12:45 PM - Post#2875107
In response to biker
As usual, we often miss the basics and drift off. Lol
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 someotherguy
Senior Moderator
Posts: 29816
Loc: Texas
Reg: 08-01-03
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11-27-24 06:42 AM - Post#2875939
In response to biker
Good job on re-checking the base timing.
As far as the heat riser flap in the air cleaner, the wax pellet has likely failed, which is pretty common. I'm not in a really cold area so I've simply removed the assembly from the air cleaner housing.
Richard
| 06 Silverado ISS / 06 Silverado SS / 06 300C SRT8 / 93 C3500 dually |
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