Stumbling off idle - Pirate4x4.Com : 4x4 and Off-Road Forum

k5blazerkeith · 05-25-2005, 11:58 AM

Truck Info: 1987 K5 with TBI 350...auto.

My truck has just started doing this after a trip to the Hammers. Basically, it runs ok, but when you go to accelerate, it stumbles and sputters off idle...as you slowly build speed, it then seems to clear out and runs fine at speed...all seems to be just off idle.

Now, some background - the last trip to Hammers saw my gas tank getting crushed by a large rock...I was able to drive the truck home (150 miles or so) with no issues. But next time I drove it, this all started. Now, I replaced the fuel tank this past weekend, replaced fuel filter...fuel pump seems to be fine. Maybe it is coincidental with the gas tank getting crushed, but truck was running fine before that trip??

Anyone have any ideas? I was told to check the TPS sensor on TBI, anyone know how to do this? I know it is something with checking voltage, but never done it...could it be a vacuum leak? Timing? I'm hoping to get the truck running well enough to make it to the mountains this weekend...

Thanks for any help you can give me!

k5blazerkeith · 05-25-2005, 03:13 PM

No ideas??

TatorZuk · 05-25-2005, 04:49 PM

Quote:

Originally Posted by k5blazerkeith

No ideas??

Before you get carried away with anything else...try unhooking the vac line to the EGR...then go test drive...see if there is a difference in the problem.

Try this first...then I'll tell you how to check the TPS.

k5blazerkeith · 05-25-2005, 04:52 PM

Ok, will do...replaced the EGR a few years ago, but those things seem to go bad a lot...thanks. I'll post up my results tomorrow...I assume that this will just set a SES light, will it affect driveability in any other way??

reddman · 05-25-2005, 05:47 PM

I would put $ on it being the engine coolant temp sensor. they are 10 bucks and will fix damn near any problem a tbi motor has.

mj · 05-25-2005, 05:55 PM

throttle tip in is a Manifold Pressure change and a Throttle Position change
start there

TNToy · 05-25-2005, 06:09 PM

A blow to the bottom of the tank could've knocked some crud loose and partially clogged the fuel filter. Just a thought.

Start with the suggestions from the guys who know bowties though. I dunno jack about TBI.

reddman · 05-25-2005, 06:42 PM

Quote:

Originally Posted by k5blazerkeith

I was told to check the TPS sensor on TBI, anyone know how to do this? I know it is something with checking voltage, but never done it...

its basically the same thing as the volume knob on your home stereo. as it rotates, the resistance goes from 0 ohms at one end, up to a set point at the other end.

stick an ohmeter on there and watch what the resistance does as you open the throttle. if it has a dirty patch on the resistor film, then it could cause an open circuit between the slider and the rest of the circuit.

when checking the resistance it will most likely be constant between the 2 outer leads on the plug, and will vary between the center lead and the 2 side leads (as its rotated) only.

mj · 05-25-2005, 07:10 PM

the joys of computer controlled crap

k5blazerkeith · 05-26-2005, 11:17 AM

Well, I fooled with the EGR and TPS stuff, no difference. On a whim, I decided to check the timing...as I went to adjust it, the distributor hold down bolt was loose...checked the timing, it was about 6-7 degrees retarded! I know I have heard that these computer controlled motors are supposed to be set to "0" degrees, then plug the computer wire back in and it takes it from there.

However, the motor ran better with a bit more timing...it ended up feeling best at 5-6 degrees advanced...is this ok? I went and drove it and it was like night and day! I could almost chirp the tires from a standstill (which is a major feat in this near 7000lb K5 with 38.5s!)

No more stumbling, no more hesitation...more power all around.

So, can I safely run with the 5-6 degrees advanced timing with no issues? Do I have to run Super Unleaded or anything? Thanks guys, it seems to be running great now...

05-25-2005, 11:58 AM	#1 (permalink)
k5blazerkeith Registered User Join Date: Jun 2004 Member # 32546 Location: Huntington Beach, CA Posts: 197	Stumbling off idle Truck Info: 1987 K5 with TBI 350...auto. My truck has just started doing this after a trip to the Hammers. Basically, it runs ok, but when you go to accelerate, it stumbles and sputters off idle...as you slowly build speed, it then seems to clear out and runs fine at speed...all seems to be just off idle. Now, some background - the last trip to Hammers saw my gas tank getting crushed by a large rock...I was able to drive the truck home (150 miles or so) with no issues. But next time I drove it, this all started. Now, I replaced the fuel tank this past weekend, replaced fuel filter...fuel pump seems to be fine. Maybe it is coincidental with the gas tank getting crushed, but truck was running fine before that trip?? Anyone have any ideas? I was told to check the TPS sensor on TBI, anyone know how to do this? I know it is something with checking voltage, but never done it...could it be a vacuum leak? Timing? I'm hoping to get the truck running well enough to make it to the mountains this weekend... Thanks for any help you can give me!

05-25-2005, 05:47 PM	#5 (permalink)
reddman Registered User Join Date: Dec 2004 Member # 39492 Location: SL,UT Posts: 2,334	I would put $ on it being the engine coolant temp sensor. they are 10 bucks and will fix damn near any problem a tbi motor has. Last edited by reddman; 05-25-2005 at 05:48 PM.

05-25-2005, 06:09 PM	#7 (permalink)
TNToy Registered User Join Date: Jul 2001 Member # 5776 Location: Memphis, TN Posts: 11,737	A blow to the bottom of the tank could've knocked some crud loose and partially clogged the fuel filter. Just a thought. Start with the suggestions from the guys who know bowties though. I dunno jack about TBI. __________________ [size=1]ASE Certified Master Automotive Technician Toyota Certified Expert Technician Volvo Certified Expert Technician Suzuki Certified Master Technician IDPA Certified Master Cardboard Killer[/size]

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05-25-2005, 03:13 PM	#2 (permalink)
k5blazerkeith Registered User Join Date: Jun 2004 Member # 32546 Location: Huntington Beach, CA Posts: 197	No ideas??

05-25-2005, 04:52 PM	#4 (permalink)
k5blazerkeith Registered User Join Date: Jun 2004 Member # 32546 Location: Huntington Beach, CA Posts: 197	Ok, will do...replaced the EGR a few years ago, but those things seem to go bad a lot...thanks. I'll post up my results tomorrow...I assume that this will just set a SES light, will it affect driveability in any other way??

05-25-2005, 05:55 PM	#6 (permalink)
mj Registered User Join Date: Oct 2000 Member # 1919 Location: BC Canada Posts: 7,547	throttle tip in is a Manifold Pressure change and a Throttle Position change start there

05-25-2005, 07:10 PM	#9 (permalink)
mj Registered User Join Date: Oct 2000 Member # 1919 Location: BC Canada Posts: 7,547	the joys of computer controlled crap

05-26-2005, 11:17 AM	#10 (permalink)
k5blazerkeith Registered User Join Date: Jun 2004 Member # 32546 Location: Huntington Beach, CA Posts: 197	Well, I fooled with the EGR and TPS stuff, no difference. On a whim, I decided to check the timing...as I went to adjust it, the distributor hold down bolt was loose...checked the timing, it was about 6-7 degrees retarded! I know I have heard that these computer controlled motors are supposed to be set to "0" degrees, then plug the computer wire back in and it takes it from there. However, the motor ran better with a bit more timing...it ended up feeling best at 5-6 degrees advanced...is this ok? I went and drove it and it was like night and day! I could almost chirp the tires from a standstill (which is a major feat in this near 7000lb K5 with 38.5s!) No more stumbling, no more hesitation...more power all around. So, can I safely run with the 5-6 degrees advanced timing with no issues? Do I have to run Super Unleaded or anything? Thanks guys, it seems to be running great now...