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Discussion Starter · #1 ·
Okay, so my 95 k2500 is surging ive replaced the fuel filter, distributor cap and rotor, spark plugs and now the tps and it’s still surging, when I get up to speed it runs fine but it will cut out when I let off the gas. When I go into reverse it will get super close or will stall out and it does the same when I go to slow down or come to a stop but it won’t stall but it will get super close. I’m going insane trying to find the problem child on this truck and idk what to change next.
 

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2000 Silverado Z71 4x4 5.3L 460k+ miles w/ GM rebuilt motor and trans
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I replied to a comment on another thread, but should probably just keep it all here.

What is your fuel pressure? Is it in spec?
 

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Discussion Starter · #3 ·
Well with this truck you can’t test fuel pressure but, I think I did find the source it’s super uncommon and most people don’t think about it off the bat ,but if I can I’m going to try and add a video to this, and if there’s no sound heres the explanation. So me and my roommates are looking at the vacuum lines and and we didn’t think to check the vacuum line leading to the brake booster and we took the line off the check valve and it wasn’t the check valve. We think it’s the break booster it’s self, since when you go to take the vacuum line off the booster the truck stops surging immediately and idles how it should be. yes there is a cold air intake on the truck. What my roommate is holding is the vacuum line to the break booster.
053D7678-7258-4D38-BEDF-7D9F14FAFD15.png
 

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2000 Silverado Z71 4x4 5.3L 460k+ miles w/ GM rebuilt motor and trans
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This is actually not the first time I've heard of the brake booster screwing with the vacuum. Good job!
 

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Discussion Starter · #5 ·
Well, it did help but it’s still surging barley it’s running a lot better but I’m going to try and replace the vacuum line to the new booster, it’s still wanting to cut out when. I slow down and when I put it in reverse
 

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2000 Silverado Z71 4x4 5.3L 460k+ miles w/ GM rebuilt motor and trans
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If you can get an inline fuel pressure tester (my cheap kit from NAPA had the attachments), you may find the underlying cause that was exaggerated by the failing brake booster.
 

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Discussion Starter · #7 ·
I might have to do that, I used a scan tool and it said the map sensor was bad or the circuit is to High and the map sensor is hooked in electrically and by vacuum and I checked the line and it’s fine. I’m wondering if it’s the sensor since while driving the check light will come on and the truck will idle normally no surge at all but the second i go to show down it wants to cut out or when the light comes off it surges
 

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Discussion Starter · #9 ·
Okay I’m about to get the fuel pressure test kit but I did think of something though, could it be bad injectors like they might be going out but look completely fine do you think that would be the cause
 

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Discussion Starter · #10 ·
The truck is running rich I did a alldata test for timing and the timing isn’t off, and with it running rich that rules out it being, fuel pump and injectors, with it have a new map sensor. So now it’s the question of what do I need to look for that’s making it run rich since the reason it’s surging is because the computer is saying it needs more fuel but when you take the vacuum line off the brake booster it’s evening out and idling properly,
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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Without actual testing the fuel pressure, you really can’t rule it out.
 
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The truck is running rich I did a alldata test for timing and the timing isn’t off, and with it running rich that rules out it being, fuel pump and injectors, with it have a new map sensor. So now it’s the question of what do I need to look for that’s making it run rich since the reason it’s surging is because the computer is saying it needs more fuel but when you take the vacuum line off the brake booster it’s evening out and idling properly,
The ECU changes A/F ratio based on engine conditions, sensors, special conditions. There are two things it has no feedback and just assumes are correct - fuel pressure and timing. This is why you want to be sure those two things are right before trying to hunt down problems. It can be confusing due to the ECU attempting to adapt to whatever is happening. You can be rich in some conditions and lean in other conditions. If you really want to debug these things you need to get something to datalog so you can watch O2 sensor, A/F cell, etc. Check out gearhead-efi for a taste of the depth of detail you can get into.
As an example of how marginal fuel pressure can mess with engine, I had a surge @ idle - 500-1k rpm back and forth on my '93. Drove OK otherwise except late/hard shifts. Chased all sorts of theories, changed parts, finally downloaded Tunerpro so I could datalog and saw that at idle engine was lean enough not to maintain idle and a A/F idle enrichment mode would kick in when low enough RPM to save it, run rich, run up RPM and repeat. Then I checked fuel pressure, it was marginal - replaced fuel pump and idle surge went away as well as the late/hard shifting problem . (I had previously read some thread where Cowboy had said that low fuel pressure was a possible for someone else's shifting problems - I discounted that as I couldn't fathom how low fuel pressure would mess with shift points.. turns out it was true). Now I'm a convert on checking basics first and recommend you do same. You'll find that many people who really tune, maintain these TBIs end up putting a permanent fuel pressure gage in engine bay.
That said - In your case, how are you determining it's running rich? and at what conditions? Are you looking at O2 sensor output or smell? If engine is never getting into closed loop it defaults to a rich A/F condition. It won't get into closed loop if it can't successfully lean out/run rich and the O2 respond as expected. If you don't have a datalogger I believe there is something (at least on OBD1 ALDL) to check if engine is in closed loop. I don't know if 95s are ALDL or OBD2.
 

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Discussion Starter · #13 ·
Okay, so I did a pressure test the fuel pump is good like I said I also replaced the IAC valve and the injectors aren’t dirty or dripping their spraying properly and pushing out just fine. And the only way I know it’s running rich is because you can smell gas, when you take off vacuum lines like the vacuum line to the break booster it idles how it is supposed to be, and the truck is a obd1 so it’s kinda hard to get a code reader the only other thing that I can think of now is gaskets and for me there’s no telling what gasket it could be
 

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If you can smell unburned fuel at the exhaust, and it idles fine when you introduced unmetered air thru the brake booster hose confirms it is really rich.
How many miles?
How about the upstream O2 sensor/s signal?

I don't skimp on O2 sensors (@100k interval) since it controls the A/F ratio. It also prolongs the cat converter service life.
 
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Okay, so I did a pressure test the fuel pump is good like I said I also replaced the IAC valve and the injectors aren’t dirty or dripping their spraying properly and pushing out just fine. And the only way I know it’s running rich is because you can smell gas, when you take off vacuum lines like the vacuum line to the break booster it idles how it is supposed to be, and the truck is a obd1 so it’s kinda hard to get a code reader the only other thing that I can think of now is gaskets and for me there’s no telling what gasket it could be
What was pressure?
check if you ever get into closed loop. Goggle ALDL closed loop or the like to find how to check. If you aren’t getting into closed loop you’ll run rich.
Tunerpro is freeware/shareware For data logging OBD1. You can buy cable from Moates or make one. Gerarhead-Efi or TunerPro forum should have info
 
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