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Chevy Throttle Body Question

2K views 17 replies 7 participants last post by  fatblock 
#1 ·
I have a '94 chevy pickup, with a 305 V8, throttle body, ect. Am having a stalling problem. It happens most when I go to back up somewhere, then, when I hit the brakes to stop to put it in drive, it will stall out, or at least run rough for a few seconds, then it is fine. When it stalls out, it always starts right back up, as if there was no problem. Seems to run rough if you take a sharp curve too quick, too. last week I was coming down a steep hill, and went to stop on the hill, it stalled out as soon as the truck came to a stop, before I put it in park. this morning it started running rough as I was going UP the same hill as I took my foot off the gas for a second.

is there a sensor or something malfunctioning on the throttle body? bad injectors? this is my first truck with a throttle body on it, always had carbs before, so I don't know much at all about them. Is there some kind of idle sensor on it, wonder if that could be it.

thanks in advance for any help.
 
#7 ·
Your EFI computer inputs are based upon the MAP (Manifold Absolute Pressure). Any vacuum leak will show up as a change in engine response/performance. The application of vacuum powered brakes is a demand on the overall vacuum being created by air flowing through the air valve. The first suspect is a leaking booster, but you need to check for other vacuum leaks to make sure you are not unnecessarily replacing your booster. It could well be the leak is elsewhere and you are running at a marginally low vacuum and the application of the brakes momentarily draws it below the minimum require for correctly operating the EFI thus an engine stumble. I would invest in a good vacuum meter and hook it inline with the MAP sensor and watch what is happening to the vacuum. I do not know what the minimum operating vacuum level is for your system, but you can find that out and go from there.

Trees
 
#9 ·
ChevyJim said:
I have a '94 chevy pickup, with a 305 V8, throttle body, ect. Am having a stalling problem. It happens most when I go to back up somewhere, then, when I hit the brakes to stop to put it in drive, it will stall out, or at least run rough for a few seconds, then it is fine. When it stalls out, it always starts right back up, as if there was no problem. Seems to run rough if you take a sharp curve too quick, too. last week I was coming down a steep hill, and went to stop on the hill, it stalled out as soon as the truck came to a stop, before I put it in park. this morning it started running rough as I was going UP the same hill as I took my foot off the gas for a second.

is there a sensor or something malfunctioning on the throttle body? bad injectors? this is my first truck with a throttle body on it, always had carbs before, so I don't know much at all about them. Is there some kind of idle sensor on it, wonder if that could be it.

thanks in advance for any help.
Are you getting the "Check Engine" light? There are a lot of sensors, wiring and connectors on top of the usual culprits of vacuum peaks, so if the computer is unhappy enough to throw codes which will be indicated by the CE light on the dash, they can help isolate the problem.

Bogie
 
#10 ·
Jim

Been dealing with this for some time now on my 94 K Blazer (5.7).
The check engine light comes on..engine starts to die. Brakes go to floor when you start to stop.
If you rev it a few times..(in nuetral)the check engine light goes off..engine runs good again.

I repalced MAP, raised idle, cleaned TBI..checked for vac leaks, replaced plugs..all the usual. A little help..but still would occurr randomly.
No OBD so no codes to read..intermitent. I'm not great at modern FI stuff. This has gone on for a year or so..intermitently.

Last week..it got a little worse...and then no start.
Dead fuel pump.
Replaced pump and filter..(naturaly day after filling tank :spank: )so far has not re-occurred..still waiting to see.

Perhaps the pump when dying..has low pressure moments that cause low idle..low vacuum. When you hit brakes and demand more vacuum..it dies.

Don't know yet..but guessin'...

Only a few hundred miles..but so far good.
 
#11 ·
Docc said:
Jim

Been dealing with this for some time now on my 94 K Blazer (5.7).
The check engine light comes on..engine starts to die. Brakes go to floor when you start to stop.
If you rev it a few times..(in nuetral)the check engine light goes off..engine runs good again.

I repalced MAP, raised idle, cleaned TBI..checked for vac leaks, replaced plugs..all the usual. A little help..but still would occurr randomly.
No OBD so no codes to read..intermitent. I'm not great at modern FI stuff. This has gone on for a year or so..intermitently.

Last week..it got a little worse...and then no start.
Dead fuel pump.
Replaced pump and filter..(naturaly day after filling tank :spank: )so far has not re-occurred..still waiting to see.

Perhaps the pump when dying..has low pressure moments that cause low idle..low vacuum. When you hit brakes and demand more vacuum..it dies.

Don't know yet..but guessin'...

Only a few hundred miles..but so far good.
Yeah I just lived thru that with my 350 S15 but it was all of sudden. Drove over to Starbucks for coffee on a Sunday morning. Went to leave, would crank but not fire. God's way of telling you that you ought to have gone to chuch, I guess.

Bogie
 
#12 ·
thanks fellas.

hey, I tried the plugging the booster off thing, out on a dirt road , and it didn't make it any different. after gunning it in forward and reverse, it still did it.

that was one of my first guesses, the fuel pump, but then the napa guy told me there was an "idle sensor" on the throttle body, so then I wasn't sure.

I have yet to see a check engine light.

I think maybe I'll try a new fuel pump, I haven't had the truck long, I don't think it will hurt. Will thow in a fuel filter as well, it just seems as though its not getting the fuel it needs when it needs more. It always seems to happen, right when I floor the foot feed, or right after. It did stall that one time coming down that hill, as soon as I came to a stop.

maybe I'll throw that sensor on too, after the pump, if the pump doesn't do it. It was only like 30 bucks.

I'll report back after these "fix its" and let you guys know if that does it or not.
 
#17 ·
va4cqd said:
well, if it was running with a badly clogged fuel filter for some time, the pump certainly could have gotten damaged and got weak
The pump depends upon fuel around it and passing through it for cooling. If you have a plugging filter that reduces flow flow, in the process allowing the pump to heat up. Also, if you tend to run the tank low, that leaves the pump exposed and it heats up. The heat eventually adds or suddens the wear process and the thing dies.

But like all things they eventually croak so as you get around a 100 kilo miles on the pump, you probably should think about replacing it.

Bogie
 
#18 ·
I have seen more than one intank pump knocked of it lower support by the baffle coming loose.The pump then hangs off the rubber hose connection and can swing up and out of the fuel in hard corners/braking etc.If you fill the tank full..this issue will disappear.Does you fuel level affect the stalling?If so,keep this in mind.
 
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