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rough idle/stumbling problems

87K5TBI

87K5TBI

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Gresham, OregOn
ok... I just put a new (stock) gm crate motor into my 87 blazer. It was running great for awhile and now runs ok when Its cold, but once it warms up it has a rough idle and bogs really bad off idle but picks up and takes off somewhere around 2k rpms. The old engine had been run with a blown head gasket for awhile, so I cleaned everything from the old engine before I put it on the new one as best as I could. So far I have the new engine, new cap/rotor/plugs/wires/o2 sensor. What do you guys think?
 
readymix said:
TPS- Could be
IAC- I doubt it
EGR- I doubt it
EST- Could be
O2- ?
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anyway to test to rule out one or the other? I really dont want to take it to a mechanic lol
 
I had a helluva time with my new engine in my 87 and it turned out that I had a vaccuum leak in one of the small hoses that goes to the MAP sensor. It is the little black box above the passenger side valve cover.
 
mtrdrms said:
I had a helluva time with my new engine in my 87 and it turned out that I had a vaccuum leak in one of the small hoses that goes to the MAP sensor. It is the little black box above the passenger side valve cover.
Click to expand...

What he said. I have zero experience with injected trucks, but I've had several instances of old carbed 350s with the exact same symptoms...and it's usually a vacuum leak... either a bad or missing line or intake gasket issues.
 
Desert Yeti said:
What he said. I have zero experience with injected trucks, but I've had several instances of old carbed 350s with the exact same symptoms...and it's usually a vacuum leak... either a bad or missing line or intake gasket issues.
Click to expand...
thanks for the info... anybody know where I could find an 87 350 tbi specific vaccum diagram? all the ones people have shown me are for different years, or a carbed version.
 
don't have a diagram handy, but it's really easy.

If you are looking directly at the TBI unit from the front. The left output goes to the EGR and AIR. The large middle output is for PCV. The smaller middle one goes to the EVAP canister. If there is one on the right, it goes to the air cleaner for TVS. The one in the rear goes to the MAP sensor.

http://coloradok5.com/forums/showthread.php?t=157677
 
well everything is hooked up right... you think the egr could be sticking open and causint a leak?
 
Could be that or the EGR solenoid is stuck open too....

You can test the EGR valve yourself. If you feel inderneath the clamshell portion , you will find that you can push the EGR valve open. When you release, it should come slowly back to the closed position. It should move smoothly.

The EGR solenoid should be closed when de-energized, so you can check it by blowing through the valve. You shouldn't be able to at all, unless you apply 12V across the solenoid terminals. Make sure you blow into the correct side though - the port that points toward the front of the motor. The other side is vented to the atmosphere when the solenoid is deenergized to allow the vacuum to vent off, and have the EGR valve close.

Have you checked your coolant temp sensor? Is it installed? A lot of problems with warmed up engines have been traced to a bad CTS.
 
btolenti said:
Could be that or the EGR solenoid is stuck open too....

You can test the EGR valve yourself. If you feel inderneath the clamshell portion , you will find that you can push the EGR valve open. When you release, it should come slowly back to the closed position. It should move smoothly.

The EGR solenoid should be closed when de-energized, so you can check it by blowing through the valve. You shouldn't be able to at all, unless you apply 12V across the solenoid terminals. Make sure you blow into the correct side though - the port that points toward the front of the motor. The other side is vented to the atmosphere when the solenoid is deenergized to allow the vacuum to vent off, and have the EGR valve close.

Have you checked your coolant temp sensor? Is it installed? A lot of problems with warmed up engines have been traced to a bad CTS.
Click to expand...

it is installed and Id assume that its ok... the temp gauge works ? I'll checkout the egr thisweekend :D thanks for the help man I really am thankfull :bow:
 
There are actually two coolant temperature sensors. One is on the drivers side cylinder head - that controls the temp guage in the dash. There is another one on the front of the intake manifold (either on the thermostat housing or next to it). It has two wires going to it - black and yellow, I believe. This sensor is just for the ECM. It helps it adjust the A/F ratio, idle, etc. based on the engine temp. Faulty CTS's seem to be the cause of many drivability problems in TBI trucks. I just changed mine and it was only 15 bucks.
 
btolenti said:
There are actually two coolant temperature sensors. One is on the drivers side cylinder head - that controls the temp guage in the dash. There is another one on the front of the intake manifold (either on the thermostat housing or next to it). It has two wires going to it - black and yellow, I believe. This sensor is just for the ECM. It helps it adjust the A/F ratio, idle, etc. based on the engine temp. Faulty CTS's seem to be the cause of many drivability problems in TBI trucks. I just changed mine and it was only 15 bucks.
Click to expand...
hrm... I'll deffinately change it then... I gott another ? for ya... my oil pressure gauge pegs itself and holds everytime I start the truck? is it just a bad sensor?
 
btolenti said:
Could be that or the EGR solenoid is stuck open too....

You can test the EGR valve yourself. If you feel inderneath the clamshell portion , you will find that you can push the EGR valve open. When you release, it should come slowly back to the closed position. It should move smoothly.

The EGR solenoid should be closed when de-energized, so you can check it by blowing through the valve. You shouldn't be able to at all, unless you apply 12V across the solenoid terminals. Make sure you blow into the correct side though - the port that points toward the front of the motor. The other side is vented to the atmosphere when the solenoid is deenergized to allow the vacuum to vent off, and have the EGR valve close.

Have you checked your coolant temp sensor? Is it installed? A lot of problems with warmed up engines have been traced to a bad CTS.
Click to expand...

His rig shouldn't have an EGR solenoid (not until later model years). I have the exact same engine/intake setup. My EGR is run completely off vaccum. I have been through this many times. What he is describing is exactly what will happen when the EGR goes bad. It will run like crap until you get above 2000 rpms. If it is the EGR, do yourself a favor and replace it w/ an OEM unit from the dealer. The aftermarket ones suck. I have never had an aftermarket EGR last more than 2 years before it crapped out.
 
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