TBI fuel injection issue...

[Ed Wright]

The O2 sensor has about zero effect on cold starts. Those older TBI trucks do not have heated O2 sensors, thus don't go into Closed Loop nearly as soon as later models. Sounds more like when it goes into Closed Loop it is correcting it's self. If it was the O2 sensor it would be more prone to go rich after it went into closed loop. Not on cold starts.
First, I would pull the air filter, and have somebody turn the key on, and watch for leaking injectors.
Next, with the truck "cold soaked", (having sat for 8 hours or so), hook up your scan tool. Just turn the key on. Verify the displayed temp is very close to the ambient temp. If it indicates a good bit colder than actual temp where the truck is sitting, that is likely your problem. Seen this many times. Same effect as the choke set too tight on a carb. When it goes into Closed Loop it will lean it's self back to 14.7-1. If the indicated coolant temp is skewed low, it will also go into closed loop later than normal, causing the rich condition to last longer.
High resistance in the coolant temp circuit will cause lower than actual temps seen by the ECM. Unplug the coolant sensor, the ECM "sees" -40 degrees F.

High fuel pressure will cause the same thing. This will also cause low BLM & INT numbers on your scan tool in closed loop. Neutral values for those parameters would be 128. Lower numbers indicate pulling fuel to correct for a perceived rich condition. Higher indicates adding fuel for a perceived lean condition.

[Alan Roehrich]

Originally Posted by Ed Wright

The O2 sensor has about zero effect on cold starts. Those older TBI trucks do not have heated O2 sensors, thus don't go into Closed Loop nearly as soon as later models. Sounds more like when it goes into Closed Loop it is correcting it's self. If it was the O2 sensor it would be more prone to go rich after it went into closed loop. Not on cold starts.
First, I would pull the air filter, and have somebody turn the key on, and watch for leaking injectors.
Next, with the truck "cold soaked", (having sat for 8 hours or so), hook up your scan tool. Just turn the key on. Verify the displayed temp is very close to the ambient temp. If it indicates a good bit colder than actual temp where the truck is sitting, that is likely your problem. Seen this many times. Same effect as the choke set too tight on a carb. When it goes into Closed Loop it will lean it's self back to 14.7-1. If the indicated coolant temp is skewed low, it will also go into closed loop later than normal, causing the rich condition to last longer.
High resistance in the coolant temp circuit will cause lower than actual temps seen by the ECM. Unplug the coolant sensor, the ECM "sees" -40 degrees F.

High fuel pressure will cause the same thing. This will also cause low BLM & INT numbers on your scan tool in closed loop. Neutral values for those parameters would be 128. Lower numbers indicate pulling fuel to correct for a perceived rich condition. Higher indicates adding fuel for a perceived lean condition.

Ed has this one nailed. The engine ignores the O2 sensor on cold start, and until it goes to closed loop.

Could be any of these, or a combination of any of these:

Leaking injectors, leaking fuel pressure regulator, or possible problem in the coolant sensor or harness.

[1legjerry]

Try reset your IAC
http://www.ehow.com/how_7959677_rese...le-gm-tbi.html

Might want to invest in a data logger if you plan to keep it for much longer
http://www.moates.net/


Good forum on how to tune a TBI.
http://www.gearhead-efi.com/Fuel-Injection/forum.php

Wish I had a 454 in my 89 V2500.

[Greg Reimer 7376]

OK. Without picking up a wrench, this is what I have suspected/deduced:.
A. Oxy sensor doesn't work on anything below a certain temperature. Since this is a cold start issue, it can't be that, since the ECU doesn't receive exhaust temperature data until at least partial warm up.
B.Engine coolant temperature sensor has been replaced. This is the two wire sensor located in front of the thermostat on the intake.If this is disconnected and the engine started it is so rich that it won't even continue running. With it plugged in, it at least runs. Obviously,this has to be working since engine warm up has a fairly fast effect on the rich condition.
C. With the air cleaner off, and the engine running, two fairly distinct cone shaped showers of fuel can easily be seen leaving the injector nozzles and making their way past the throttle blades into the engine. Disconnecting each injector's electrical connector one at a time and reconnecting them results in an equal drop in rpm as the engine receives less fuel.
D. With the engine not running,activating the fuel pump to pressurize the fuel system didn't reveal any injector leakage.
E.All of the above described suggestions and discussion are relevant, I just haven't taken the time to find it yet. I'm sure the catalytic converter that I took off was ruined by cold starts and too much fuel. This truck only ran 2000 miles in the last 4 years since I got it. It runs well and tows strong. I went to Bakersfield two or three years ago towing my open trailer and Chevelle up there, a distance of 150 miles each way,
you all know that that's not exactly an economical flat ride, and got 10 miles per gallon, doing the whole trip on less than one tank of fuel. I'll get it right, and update this thread.

[Ed Wright]

There is more to it than simply replacing the coolant temp sensor. I have fixed many like this over the years by cleaning or correcting loose/high resistance connections in that circuit, at both ends. As I mentioned, high resistance = lower than actual temp indications to the ECM. That is the reason for cold soak checking the coolant temp with your scan tool. Trying to work on an EFI vehicle without a good scan tool is like trying to work on a carb with no screw driver.

Another thing I forgot to mention is the tendency of those older TBI trucks to clog (with black. carbon/coke) the vacuum passage to the MAP sensor hose nipple in the back of the TBI, causing lazy response of the MAP sensor. If the ECM is seeing lower than actual manifold vacuum causes it to mis-calculate load, and reference cels in the wrong area of the V.E. Table, and making it run too rich. This is a mileage killer, for sure. A kinked, or leaking vacuum hose to the MAP sensor will cause that also.
I have seen many more low coolant temp indications cause cold start up problems in my shop than anything else.

[ss3011]

If you don't have access to a Tech1 or a SnapOn Scanner, You can get a decent scanner cheap from Harbor Freight, amazing how low the price is , if you have a discount coupon !

[Mark Ugrich]

The problem could be any of the things suggested. Using a scan tool and checking for codes will help guide you in the right direction.There are diagnostic aids for each fault code in the service manuals.