1991 r3500n 454 cfi, cold start issues....

[cag]

this cfi 454 on start up runs really rough and backfires sometimes, any idea what is going on???

[Bob B.]

When you shut the engine off, is there fuel leaking from the injectors flooding the intake manifold? Also check to make sure the engine coolant temp. sensor (the one for the computer, not the gauge) is reading properly. You can check it with a scan tool. Sometimes they can be way off and cause hard start problems. Does in run O.K. once it warms up?

[kwmech]

1991 big block is going to be a TBI so first I would check the EGR spacer underneath the throttle body for vacuum leaks. Then I would move on to the coolant temp sensor by the t-stat and make sure the wiring is not messed up. After that?? maintenance for a tune up, plugs wires? what shape are they in. Plugged fuel filter etc.

[cag]

Originally Posted by Bob B.

When you shut the engine off, is there fuel leaking from the injectors flooding the intake manifold? Also check to make sure the engine coolant temp. sensor (the one for the computer, not the gauge) is reading properly. You can check it with a scan tool. Sometimes they can be way off and cause hard start problems. Does in run O.K. once it warms up?

it runs fine after it warms up, will look at injectors, what scan tool?

[cag]

Originally Posted by kwmech

1991 big block is going to be a TBI so first I would check the EGR spacer underneath the throttle body for vacuum leaks. Then I would move on to the coolant temp sensor by the t-stat and make sure the wiring is not messed up. After that?? maintenance for a tune up, plugs wires? what shape are they in. Plugged fuel filter etc.

plugs, wires,coil,dist all brand new.

[kwmech]

So that leaves us with a vacuum leak or a CTS and/or black and yellow wiring at the connector.

[dieseldawg142]

try the vacuum line from your MAP.
my hose had a small hole in it and my 454 ran like sheet till i found it.
just throwing something out there...

[cag]

Originally Posted by kwmech

So that leaves us with a vacuum leak or a CTS and/or black and yellow wiring at the connector.

does the wiring go bad?

[cag]

Originally Posted by dieseldawg142

try the vacuum line from your MAP.
my hose had a small hole in it and my 454 ran like sheet till i found it.
just throwing something out there...

the small black box behind the egr?

[kwmech]

Originally Posted by cag

does the wiring go bad?

With the way the connector is oriented at the front of the intake, the wires hang off the top of the sensor and after years of this the wires get stressed and start to separate from the connector and the insulation exposes the bare wires to the elements. A new pigtail can be had at your local NAPA store if needed

EDIT: Sensor will be near the t-stat housing and have yellow and black wires coming out of the loom to the connector

[cag]

just had it run on a scanner, temp sensor is reading correct, it is super rich when warming up

[kwmech]

Check the MAP sensor or for a vacuum leak

[cag]

replaced the map and the vacuum line to it, no more backfiring on start up, thanks for all the help!!

[chris989]

download WINaldl.. it is free,, only need a cable to see in real time what is going on or data log while you drive.

https://winaldl.joby.se/