95 350 Doesn't Like Low Speed

[95C1500]

I've had this issue for months now. Just recently started looking into it. Stupid on my part, but what's done is done.

For starters, you know the sound a cammed truck makes? Lopey until you get on the gas then it seems to smooth out? Mine is the opposite. It seems to idle fine but give it a little throttle and it's choppy. Feels like it's bucking. Like it wants to die but it catches itself right before it does... over and over and over... Let off the gas and you can still hear and feel the lope/choppiness. And by low speed, I'm talking sub 10 mph. Think drive thrus, stop and go traffic, and small parking lots. Also, backing up an incline. Oddly specific but that's another time that, without fail, this happens. One night, I was idling in a drive thru and the check engine light came on. When I got home, I pulled a code 44 - Lean 02 code. CEL went away after I started driving and I haven' seen it since. Give it a decent amount of throttle and traveling at >10 mph and it seems to run fine. I think it's also worth mentioning that some mornings the truck will start, idle for a second and the die. If I give it some time, it'll start up and idle fine. It will also usually start up fine if I give it some throttle right after starting it. Not sure if it's related but I think it might be. Does it when it's warm too... sometimes.

So I've read countless threads and watched countless videos about the issue. Here's what I've done so far.

-Checked for vacuum leaks using the carb cleaner method and that turned up empty
-I replaced the fuel an air filters
-Tested the 02 sensor. It varied from 0.02v to 0.79v instead o 0.4-0.6 like it was supposed to. Replaced
-Tested the map. I forget which pin, but it tested as 0.17v instead of 0.5v. Replaced
-TPS tested fine
-Pulled the dist cap off to check for anything out of the ordinary for 49,000 miles. I'll post the pictures below because, honestly, not exactly sure what I'm looking at as fa as normal wear goes.

I don't have a vacuum pump so I have not been able to test the egr system
I bought a fuel pressure gauge to install in the engine compartment but have yet to do so.

I know fuel pressure is kind of important to know in this situation but let's assume, for the sake of this thread going somewhere helpful, that fuel pressure tested fine.

Here's the dist cap

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Rotor

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I'm about ready to push the truck off a cliff so, to save the truck, what are some other things I should look into?

 

[MSCustoms]

Did you check your 02 and map sensors after they were replaced to verify the numbers changed? Another way to test the egr is to unplug the vacuum line, manual compress the diaphragm and hold your finger over the port. That essentially creates a vacuum that the egr valve should stay open for

 

[MSCustoms]

How does the injector spray look too? When you take off the air filter and sleeve do the injectors have a uniform spray or have dripping?

 

[Gramps]

That cap doesn’t look too overly terrible but not good either. And the rotor looks like crap. Also you make no mention of the wires/plugs condition. First thing I’d do is a simple tune up. Cap rotor plugs and wires. Make sure to get the premium cap and rotor with the BRASS contacts. NOT the cheapo with the aluminum. Ac Delco plugs are fine, they are cheap and they work.
Then I’d start looking into other problems -simple enough look at the spray pattern of the injectors as previously mentioned. Nice strong Christmas tree shaped pattern with no dripping.

 

[PlayingWithTBI]

IAC, MAP, and TPS sensors tell your ECM what's going on. Then it sends more or less spark or fuel. As suggested above, simple tune up then initial set up of your TB. This is an old article but is still germane to your system. Make sure your initial timing is set with bypass wire disconnected too.

http://www.gmcmidwestclassics.org/Web pages/Tuning the TBI.pdf

 

[95C1500]

Did you check your 02 and map sensors after they were replaced to verify the numbers changed? Another way to test the egr is to unplug the vacuum line, manual compress the diaphragm and hold your finger over the port. That essentially creates a vacuum that the egr valve should stay open for

I should have. I will do that tomorrow.
Thanks for the egr trick

How does the injector spray look too? When you take off the air filter and sleeve do the injectors have a uniform spray or have dripping?

I’d have to look again to say for sure but iirc it looked good. I don’t remember any dripping

That cap doesn’t look too overly terrible but not good either. And the rotor looks like crap. Also you make no mention of the wires/plugs condition. First thing I’d do is a simple tune up. Cap rotor plugs and wires. Make sure to get the premium cap and rotor with the BRASS contacts. NOT the cheapo with the aluminum. Ac Delco plugs are fine, they are cheap and they work.
Then I’d start looking into other problems -simple enough look at the spray pattern of the injectors as previously mentioned. Nice strong Christmas tree shaped pattern with no dripping.

I should have started there I guess. I replaced them at 100000, 49000 miles ago. My grandpa was the previous owner and looking through his reciepts for the truck, I never saw anything about them being replaced beforehand. I’ll order/pick up a set tomorrow as well as check the injector spray pattern

Given this is happening around idle and low speeds couldn't the IAC be misbehaving?

IAC was replaced as well. Forgot to mention that. I did no testing with that one. Hoped it would be a simple one and done deal but that wasn’t the case so it definitely could be an issue

IAC, MAP, and TPS sensors tell your ECM what's going on. Then it sends more or less spark or fuel. As suggested above, simple tune up then initial set up of your TB. This is an old article but is still germane to your system. Make sure your initial timing is set with bypass wire disconnected too.

http://www.gmcmidwestclassics.org/Web pages/Tuning the TBI.pdf

I will take a look at that article in the am. Thanks

 

[joep88cheyenne]

Yeah that rotor didn't look good IMHO. Their cheap so replace and yeah brass teonal in the cap would be a positive. I did see that white power coating on those terminals. So a scrape or replace it also.

 

[95C1500]

I have been busy the last several days and haven't had a chance to check everything. However, I figure tomorrow I can d plugs/wires/cap/rotor. The only thing is the only caps I find with brass terminals seem to be cheap off brand caps. Every AC Delco I've seen has aluminum contacts. I bought AC delco to begin with. Anyone recommend certain brands?

 

[Gramps]

Oreillys BWD is now branded “STANDARD” but same parts, and is brass; Napa “echlin” line should also be brass - granted it’s been 8 years since I worked there.
MSD streetfire is also brass.

 

[95C1500]

I ended up ordering a Taylor cap with brass terminals and ac delco plugs/wires/rotor. I'll pick those up after class and get them swapped out tonight or tomorrow morning and see if it helps.

This was before I saw your reply. If the Taylor is crap, let me know and I'll return it and pick up a Standard cap