95 P-30 Truck Missing And Stalling

ddavison · December 9, 2007

I have a 1995 chevy p30 18' box truck with a 7.4L TBI and I am experiencing some strange problems.

The truck's fuel pump went bad and was replaced along with the inline fuel filter and a new tank strainer and after replacing, the truck started but idle was cycling up and down and when I put the truck in gear, it would stall.

When I started the engine again and reved the engine slightly I would get small but rapid backfiring thru the throttle body

so I did some checking and it was suggested that I replace the fuel pump pulsator with a short piece of fuel line that comes with the fuel pump to see if that cures the problem. I did this and found no change.

I checked for vacume leaks and found no leaks anywhere and so I moved to the next step and pulled the codes and got code 54. but since this engine has a tbi the code did not make any sence.

I checked the cap and rotor which is fairly new and they both checked out ok. Now just to be safe I pulled the distributer and replaced the pick-up coil and the control module. Set the base timing to 4 degrees btdc with the single control wire unpluged as per the manuals procedure. I then cleared all codes in the ECM by disconnecting power and waiting 60 minutes (lunch hour)...

came back from lunch and reconnected the power and started the engine....engine started, idle cycled up and down and smoothed out after pedeling the throttle for a few minutes and then the idle went from normal to a fast idle and stayed that way for about 10 minuted and then I reved the engine for a moment and idle returned back to normal and then it would cycle up and down and then smooth out ....cycle a bit then smooth out.... turned of engine and pulled codes again ....got a code 35 (idle air control)

Started engine again ...idle normal ...put truck in gear ....stall.....start engine again ...put in gear again but peddled throttle to keep engine running.....engine surged and lobed and bucked.....pulled truck back into the shop and let idle. hmmmm fuel issue....ok fine grabbed a painters clamp light with a 100 watt bulb and looked at the two injectors spraying ....pattern looked fine but I noticed that if I moved the throttle to the wide open position ...the injectors stoped spraying for a moment and then started again but engine bogged as if it were leaning out and as this was happening I noticed rapid poping or backfiring up thru the throttle body at medium to high RPM....interesting

Ok... heres what I have to work with

#1... IAC

#2... Linear EGR

#3... TPS

#4... MAP

#5... ECM

Since I know that the throttle actuater is working and the fact that my main problem seems to be bogging under any kind of load, Im Going to try replacing the TPS and the MAP on this next round then if that does not cure the problem I will move on to the IAC and the linear EGR. I have a new ECM but that will only get replaced as a last resort.

Now heres something I find Interesting ... When the truck idles smooth or cycling, I noticed that the exaust smells like that of an engine dieseling or suffering from run on after the ignition is turned off .....and it really makes your eyes water and irratated.

I am quite sorry for this thread being so long winded but I thought it would help if I brought the group up to speed step by step as to the problems Ive experienced .

Thankyou for your help

Denis

[email protected]

ddavison · December 11, 2007

I have a 1995 chevy p30 18' box truck with a 7.4L TBI and I am experiencing some strange problems.

The truck's fuel pump went bad and was replaced along with the inline fuel filter and a new tank strainer and after replacing, the truck started but idle was cycling up and down and when I put the truck in gear, it would stall.

When I started the engine again and reved the engine slightly I would get small but rapid backfiring thru the throttle body

so I did some checking and it was suggested that I replace the fuel pump pulsator with a short piece of fuel line that comes with the fuel pump to see if that cures the problem. I did this and found no change.

I checked for vacume leaks and found no leaks anywhere and so I moved to the next step and pulled the codes and got code 54. but since this engine has a tbi the code did not make any sence.

I checked the cap and rotor which is fairly new and they both checked out ok. Now just to be safe I pulled the distributer and replaced the pick-up coil and the control module. Set the base timing to 4 degrees btdc with the single control wire unpluged as per the manuals procedure. I then cleared all codes in the ECM by disconnecting power and waiting 60 minutes (lunch hour)...

came back from lunch and reconnected the power and started the engine....engine started, idle cycled up and down and smoothed out after pedeling the throttle for a few minutes and then the idle went from normal to a fast idle and stayed that way for about 10 minuted and then I reved the engine for a moment and idle returned back to normal and then it would cycle up and down and then smooth out ....cycle a bit then smooth out.... turned of engine and pulled codes again ....got a code 35 (idle air control)

Started engine again ...idle normal ...put truck in gear ....stall.....start engine again ...put in gear again but peddled throttle to keep engine running.....engine surged and lobed and bucked.....pulled truck back into the shop and let idle. hmmmm fuel issue....ok fine grabbed a painters clamp light with a 100 watt bulb and looked at the two injectors spraying ....pattern looked fine but I noticed that if I moved the throttle to the wide open position ...the injectors stoped spraying for a moment and then started again but engine bogged as if it were leaning out and as this was happening I noticed rapid poping or backfiring up thru the throttle body at medium to high RPM....interesting

Ok... heres what I have to work with

#1... IAC

#2... Linear EGR

#3... TPS

#4... MAP

#5... ECM

Since I know that the throttle actuater is working and the fact that my main problem seems to be bogging under any kind of load, Im Going to try replacing the TPS and the MAP on this next round then if that does not cure the problem I will move on to the IAC and the linear EGR. I have a new ECM but that will only get replaced as a last resort.

Now heres something I find Interesting ... When the truck idles smooth or cycling, I noticed that the exaust smells like that of an engine dieseling or suffering from run on after the ignition is turned off .....and it really makes your eyes water and irratated.

I am quite sorry for this thread being so long winded but I thought it would help if I brought the group up to speed step by step as to the problems Ive experienced .

Thankyou for your help

Denis

[email protected]

UPDATE! Ok, while doing some testing, the new waterpump that I installed 2 years ago failed and because it is behind every accessory bracket that has to come off to R & R the waterpump, I figured why not put a timing chain in since the engine has 165,000 miles on it. Now when it comes to timing chains I go straight for the double roller type every time. they only cost ten bucks more that the stock style and they are a lot more durable.

Denis

ddavison · December 20, 2007

Ok ....I have the truck back together and I fired it up and it ran fine. It went thru the high idle step untill it warmed up and then it started doing a hi/low cycling and then it died. I rechecked all the vacume lines one by one and when I disconected the vacume line to the map it idled fine. when I reconected the vacume line to the map.....it stalls

any thoughts?

Denis

ddavison · December 21, 2007

Ok...... after spending some time on the phone this morning, A friend of mine called his friends older brother who owns a shop. He informed me that he just had a 350 tbi engine that displayed the same symtoms as the ones im having to deal with. according to him after he tryed everything under the sun and switched out every sensor including the computer he finally found the problem. it turned out to be an air leak between the head and the intake manifold. im in the process of that job right now and ill post when i have the results of my latest effort

Denis