tbi runs better with temp sensor unplugged...need advice

[bigarmzz]

i need some advice guys, this truck of my dads has my scratchin my head, it has a bad miss after it warms up, so first thing i thought was to change the tank, sending unit and fuel filter, after all its like 25 yrs old...felt like it wasnt getting enough fuel, ya know? like it had a clogged fuel filter, but that didnt help

its a 87 tbi, and it starts up and idles fine, after just a few minutes though it runs like crap and sputters real bad....i thought it was the temp sensor, so i put a new on on, didnt help

so i was curious last eve, took it for a spin a few miles, it was sputtering real bad and missing as usual, i pulled over and unplugged the temp sensor, and it ran fine....????

what the heck would make a tbi run better with the temp sensor completely unhooked??? i'm baffled

[BigBlocksRule]

Which temp sensor did you replace? The one in the head is for the gauge, the one by the water neck is for the computer. Mine always ran like crap if I unplugged it...

[rfmaster]

Originally Posted by bigarmzz

i need some advice guys, this truck of my dads has my scratchin my head, it has a bad miss after it warms up, so first thing i thought was to change the tank, sending unit and fuel filter, after all its like 25 yrs old...felt like it wasnt getting enough fuel, ya know? like it had a clogged fuel filter, but that didnt help

its a 87 tbi, and it starts up and idles fine, after just a few minutes though it runs like crap and sputters real bad....i thought it was the temp sensor, so i put a new on on, didnt help

so i was curious last eve, took it for a spin a few miles, it was sputtering real bad and missing as usual, i pulled over and unplugged the temp sensor, and it ran fine....????

what the heck would make a tbi run better with the temp sensor completely unhooked??? i'm baffled

Your Fuel pump is almost gone (kaput). The reason why it runs a bit better with CTS unconnected is that when CTS disconnected the open circuit corresponds to a censor seeing very cold coolant temperature -40C! Under such cold conditions ECM will add more fuel - increasing injector pulse width. However, I willing to bet that your fuel pressure is around 4 to 6 PSI (or possibly less), normal range being 9 to 13 PSI. Low fuel pressure typically results from either worn out fuel pump, clogged fuel filter or leaky in tank fuel hose between Fuel Pump and feed through.

Verify fuel pressure with a suitable FP gauge. If found to be low drop tank, inspect pump and fuel hose (and pickup-up sock). There are plethora of FP's available - a very common upgrade is to go with AC-Delco EP241 (this will support higher fuel pressure for future upgrades).

and please do not drive it until you get is squared away. When done with repairs you'll have to clear ECM codes by disconnecting battery for 30 sec.

//RF

[JCampbell]

You're so smart, RF.

[bigarmzz]

thing is though, i already put a new fuel filter on it, new fuel pump/sending unit and new fuel tank

dont know which route to go now....is there a fuel pressure regulator on the tbi?

Originally Posted by rfmaster

Your Fuel pump is almost gone (kaput). The reason why it runs a bit better with CTS unconnected is that when CTS disconnected the open circuit corresponds to a censor seeing very cold coolant temperature -40C! Under such cold conditions ECM will add more fuel - increasing injector pulse width. However, I willing to bet that your fuel pressure is around 4 to 6 PSI (or possibly less), normal range being 9 to 13 PSI. Low fuel pressure typically results from either worn out fuel pump, clogged fuel filter or leaky in tank fuel hose between Fuel Pump and feed through.

Verify fuel pressure with a suitable FP gauge. If found to be low drop tank, inspect pump and fuel hose (and pickup-up sock). There are plethora of FP's available - a very common upgrade is to go with AC-Delco EP241 (this will support higher fuel pressure for future upgrades).

and please do not drive it until you get is squared away. When done with repairs you'll have to clear ECM codes by disconnecting battery for 30 sec.

//RF

[bigarmzz]

Originally Posted by BigBlocksRule

Which temp sensor did you replace? The one in the head is for the gauge, the one by the water neck is for the computer. Mine always ran like crap if I unplugged it...

it was the sensor by the water neck

[rfmaster]

Originally Posted by bigarmzz

thing is though, i already put a new fuel filter on it, new fuel pump/sending unit and new fuel tank

dont know which route to go now....is there a fuel pressure regulator on the tbi?

Check Fuel Pressure first. It is rare, but it does happen - fuel pressure regulator spring breaks into pieces rendering FPR partially operative. Get Fuel pressure gauge and verify your fuel pressure while engine is running.

If you got one of them white box, no name Chinese made pumps - it is possible that it may have failed shortly after being installed.

//RF

[bigarmzz]

Originally Posted by rfmaster

Check Fuel Pressure first. It is rare, but it does happen - fuel pressure regulator spring breaks into pieces rendering FPR partially operative. Get Fuel pressure gauge and verify your fuel pressure while engine is running.

If you got one of them white box, no name Chinese made pumps - it is possible that it may have failed shortly after being installed.

//RF

well it was doin the same thing with the old pump/tank/sending unit as its doin now with the new stuff

the thing is, it isnt throwing any codes (no check engine lights), but if i unplug the temp sensor, it will throw a code, but runs just fine then

[rfmaster]

ECM has no way of figuring out what is the fuel pressure - there is no fuel pressure sensor in TBI system. Since ECM is not throwing any codes other than CTS is too cold when you disconnect - it would lead me to believe that you have fuel pressure or fuel delivery problem. Insomuch, pull air cleaner assembly and verify that both injectors are spraying fuel. Also take a look at injector wire harness. There are four wires used to power up this circuit - make sure that insulation is intact.

//RF

[deth_1970]

what they said about the fuel system^, I would also look at the ignition system, and a module to the left of the tbi unit, it has some vac hoses going to it, and some wires, I chased my tail on one of those and it turned out to be that module