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Low idle at start up 87 350 tbi. update, backfiring?? last post

mudboggink5

mudboggink5

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Its been getting colder, and when i start the truck up in the morning, i have to hold the gas pedal down just to get it started. When it starts its at a very low idle, and takes 30 min just to get the heat goin.
If i try to drive it cold it will pop,and try to stall out.
Ive recently changed plugs, rotor, cap, o2 sensor, and the coolant temp. sensor. I have off tomorrow, so im gonna check the other sensors, egr valve, tps, and iac. also gonna change the thermostat. any other tips.
Also did the seafoam in the tank, and through the vaccuum hose.
When i took the vaccuum line off the master to do the seafoam, it idled alot better.
 
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mudboggink5 said:
When i took the vaccuum line off the master to do the seafoam, it idled alot better.
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When your throttle shaft wears the throttle blades close and all the air is coming in from IAC. This is bad because all the gas is sitting on the throttle blades.

If you have no codes set and everything is proper AND my guess from 2500 miles away is correct, then drill out the lead block in front of the minimum air screw (Idle screw) and give it a half turn in. Try again next morning, then another half turn...

You could varify this with a scantool or laptop with cable and WinALDL.
 
just curious, which coolant temp sensor did you change out, the one in the intake or the one in the side of the head?
 
I hope it's the one in the intake near thermastat if he's chasing a EFI problem...:doah:
 
You say you recently changed the O2 sensor. Did you do it carefully and properly? My understanding is that if you use the wrong thread locker it can contaminate the sensor. A bad o2 sensor will not tell the engine that it is warmed up and cause problems, perhaps like you are seeing.

Personally, I would recommend you NOT start changing everything out but instead properly diagnose it. Jump the cables in the data port and check for computer codes. It's easy and free and will only take 5 minutes. I posted a guide at the end of the the "K5 Buyers Guide" which is stickied in the main forum.

Otherwise, when you just start randomly replacing things on a whim you typically are just throwing money away.

eagle mark said:
If you have no codes set and everything is proper AND my guess from 2500 miles away is correct, then drill out the lead block in front of the minimum air screw (Idle screw) and give it a half turn in. Try again next morning, then another half turn..
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Can you explain this? What does this fix? From all my experience with fuel injected engines, this may create a more problems rather than solving whatever is really wrong. But I dunno if you have some secret fix so please share!
 
Can you explain this? What does this fix? From all my experience with fuel injected engines, this may create a more problems rather than solving whatever is really wrong. But I dunno if you have some secret fix so please share!
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Your right but the blades do have a setting. He has a low idle at startup problem. When throttle shafts wear they close the blades loosing the minimum air setting leaving all air at idle to come through the IAC. Leaving no room to pull air and gas through the blades.
 
eagle mark said:
Your right but the blades do have a setting. He has a low idle at startup problem. When throttle shafts wear they close the blades loosing the minimum air setting leaving all air at idle to come through the IAC. Leaving no room to pull air and gas through the blades.
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what are we considering "wear" though, i am currently still using the original throttle body on my engine, with over 260k miles (on the TB unit) and haven't had any problems with it so far. OTHER items, like sensors and electronic components are the norm it seems. :doah:

It just sounds like a sensor is not working correctly to me.
 
Yeah, I will agree, my two TBI trucks have 187k and 250k miles on them and they both started up just fine today, and it's 22 degrees out! That was why I was thinking adjusting the throttle seemed odd.

It may be worn out injectors. I bought new ones for $75/each. Maybe your fuel pressure regulator? That's cheap.

However, I would CHECK CODES first. So easy to do!
 
Not uncommon to see that many miles and no SIGNS of wear. More wear on vehicles with a stick shift and more common in city than guys that drive 100's of miles across the desert without moving there foot. But it does happen, seen it more than once.

4xcrazy said:
what are we considering "wear" though, i am currently still using the original throttle body on my engine, with over 260k miles (on the TB unit) and haven't had any problems with it so far. OTHER items, like sensors and electronic components are the norm it seems. :doah:

It just sounds like a sensor is not working correctly to me.
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Like I said, just a guess and something to look at. Could actully be the IAC but could,t tell without putting a scantool on it.
 
ok. so i had the day off, and worked on the truck most of the day.
I ended up taking the tb off and cleaning it up, i checked the IAC and the thing was stiff it did not move in or out. so i changed that out, and i figured i would just rebuild the tb, new injectors and pressure reg. and of courst new gaskets.
I started it up and the motor idles perfect, much better response.
Now, my guy at napa siad i have to get the truck over 45mph in order to set the iac. is this true.
 
mudboggink5 said:
Now, my guy at napa siad i have to get the truck over 45mph in order to set the iac. is this true.
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Glad you got it fixed.:waytogo:

Sounds like the guy at NAPA has been hitting the:smokin2:.

The computer will set the IAC where it needs to be as long as it's between it's operating limits.

The only other thing you can do that is IAC related is to set the minimum air flow that eagle mark was talking about, and that isn't really necessary if it's running ok.
 
You know, ther is a MPH thing on an ECM but I can not remeber what it reset after that MPH... man I am getting old...

The ECM does have built in parameters for engine wear and this should not happen under normal operateting conditions or vehicle life. But if you ran the same TBI through 2 or 3 250,000 mile engines... the reason I bump into it is conversions of this system to other engines and setting min air is a uusual step. I like to get within 100 RPM of chip idle and about 20-40 IAC counts in WinALDL or a scantool.

Once upon a time I worked on a ford Bronco with a GM TBI running very well except the throttle was very hard to open from idle. It also had issues with cold start. when I worked on it I found the throttle shaft worn to the point the throttle blades were sealed and fuel would pool in the bores. Setting the min air cured all issues.
 
bigblock72 said:
Glad you got it fixed.:waytogo:

Sounds like the guy at NAPA has been hitting the:smokin2:.

The computer will set the IAC where it needs to be as long as it's between it's operating limits.

The only other thing you can do that is IAC related is to set the minimum air flow that eagle mark was talking about, and that isn't really necessary if it's running ok.
Click to expand...

Ok. he usualy knows what hes talking about. I did give that that little hidden screw a half turn. I think it helped.
The truck still falls alitle when i give it throttle in gear, and it picks up pretty quick compared to before. I do have a exhaust manifold leak. I think that might be the culprit.
 
I have the same year and the same problem. You described my truck to the letter.

Ill be watching this one.

I just totally rebuilt my motor and almost all accessories and am having the same problem. I gave the idle screw a quarter to half turn and it didnt really help. The truck wint start on the first start when cold. I just bought a remote start to warm the truck up before i leave home but it wont work beacause of this problem. After its warmed or has atleast moved for the day it works fine but that first start requires some skinny pedal to get it running.

New parts: egr, temp sensor, Complete longblock, map, new throttle body and injectors, etc.....Its all new. Not throwing any codes and truck runs pretty good overall. Im thinking IAC because i know its not vacuum related and hope all the new sensors are working.

And to make it even stranger....I too have a manifold leak that i can slightly hear.
 
Just because you have new parts does not mean they are good or properly installed. Not meant to be mean just the truth...

If you have a vacuum leak on a TBI engine or any EFI engine then don't expect anything to work properly becaue the ECM has the wrong information.

Cold start is I think the hardest thing to fix on EFI engines cause you only get one shot a day and even with my Scantool the slow baud rate of the pre 94 TBI engine I sometimes can't see the issue for a few times (days).

Turning the screw in was just a educated geuss. I don't usually give advice unless I know why... here is a good write up on how to properly set the minimum air setting on a TBI.
http://www.thirdgen.org/techboard/posts/tpi/131292-iac-idle-screw-must/866667-post4.html
 
New parts: egr, temp sensor, Complete longblock, map, new throttle body and injectors, etc.....Its all new. Not throwing any codes and truck runs pretty good overall. Im thinking IAC because i know its not vacuum related and hope all the new sensors are working.


Just unplug the iac, and unscrew it out, and check the spring. Mine was stiff, so i knew that was part of the problem.
 
The truck drives way better than it did. its still alitlle hesitant at low idle , and picks up much quicker than it used to.
I was still having problems with starting the truck when it was already warmed up. I figured it was the controll module, so i changed it and i think it solved that problem. After i changed the module, I started the truck, and was reving the throttle by hand. I noticed when i would give it full throttle quickly it would back fire.
What could be causing this to happen. :confused: Timing to advanced. It is definitly running rich.
 
Backfiring through the intake can be lots of things like a vacuum leak, bad intake valve or running lean. If you think about it theres no reason to give an engine full throttle at idle in park.neutral so maybe it's not programmed into the ECM even though most engines can do this.

Since you have other issues can you put a vacuum gauge on the engine? Look at idle vacuum, what is it? Is it steady?
 
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