rough idle issue help

[bigblue87]

I'm not going to lie this truck has me going nuts. So im going to give you guys the low down and hopefully i can get this problem solved....since the guys on the 73 87 dont have much experience with the tbi ill see what you have to say

87 Chevy v10 with a 5.7 engine TBI it is idling very rough and seems to be missing and running rich...seems to smooth out over 2000 rpm

thats how it runs now and how it ran before i did all this stuff:

Did a compression test and found no compression in the 8th cylinder so i tore the top half of the motor down to find it had a sucked valve and the cam and lifters were toast. Had the heads gone through at a shop and got a new set of lifters and a computer friendly cam. At this point i expected it to fire up and run like a champ....but no luck. the only thing that actually helped was the hard shift i was getting from 1st to 2nd.

ive replaced most every sensor in the thing. distrib looks fine... replaced the coil plugs & wires took a look at the tbi spray and it seemed decent except it seemed to be spraying alot more fuel than it needed (actually seemed to smooth out when blocking one injector with my finger)

Hopefully some one has some ideas cause im about ready to give up on this damn thing hahaha

[speedygonzales]

Squeeze the EGR valve while the engine is idling to see if the idle quality gets worse. chances are your EGR has crust in it staying open while it's idling. It could also be leaking at the gasket. Also try disconnecting the vac line and plugging the hose to see if it gets any better.

[ChevyTech]

My first thought:
Did you unplug the set timing connector when you set the ignition timing?

Next thing:
Anytime a TBI system runs rich you should test the coolant sensor that the computer monitors.
Check post #3 in this thread for links to specs and test procedures.
http://67-72chevytrucks.com/vboard/s...d.php?t=445487

[bigblue87]

Egr has been replaced and solenoid.
yes the timing was set with the wire unplugged.
and i never checked the coolant sensor but i replaced one... or at least the one on the top of the intake.

[speedygonzales]

What do the plugs look like?

What is the fuel pressure?

[bigblue87]

Plugs and wires are new.
Plugs have some carbon on them though.
I havent tested the pressure but the injectors seem to be sprayin pretty fierce. At idle it really seems like to much fuel.

From what i understand the is a fuel pressure regulator in the tbi and i was wondering if that is adjustable or is it computer controlled ?

[ChevyTech]

The fuel pressure regulator is in the TBI unit. From the factory it is not adjustable.

The computer does not control the fuel pressure, and on your TBI truck the pressure is designed to stay constant regardless of what the engine vacuum is.

If you have not checked the ECM for trouble codes you should. This can be done without a scan tool on your OBD1 system. There is a link to instructions in the FAQs sticky.

Make sure there is good vacuum to the MAP sensor.

[bigblue87]

Now its starting to surge at idle in gear
and its also back firing under a load when the engine gets up to temp

im wondering maybe if she doesn't like the factory set 0 timing
or maybe the cam i put in wasn't as "computer friendly" as they told me it would be.

[ChevyTech]

Originally Posted by bigblue87

Now its starting to surge at idle in gear
and its also back firing under a load when the engine gets up to temp...

This sounds like a lean mixture, not to rich like you were talking about earlier.

Originally Posted by bigblue87

...im wondering maybe if she doesn't like the factory set 0 timing
or maybe the cam i put in wasn't as "computer friendly" as they told me it would be.

Test the vacuum to the MAP senor at idle and let us know what you find.

If the timing has to be set advanced from spec it indicates a problem with the electronic spark control.

[bigblue87]

I wont have a chance to play with the truck for a few days but this electronic spark control has me wondering. from what im reading is it tells the computer to retard or advance the timing depending it senses a knock?
and if something is wrong there it could cause problems with the engine... could that be the reason for the cam and lifters being wiped out?

as far as the map sensor from previous knowledge i have pulled the vac line off and the engine has died.... not sure if that means anything to you.

thanks for all the input though i appreciate it. I just want to get this beast running cause im not going to lie i love driving it over my new car haha

[ChevyTech]

Originally Posted by bigblue87

from what im reading is it tells the computer to retard or advance the timing depending it senses a knock?

Yes

Spark control problems can greatly affect performance and gas mileage, but should not damage the cam & lifters.

Spark control problems along with a lean mixture can allow a piston to melt and the lost of compression and use of that cylinder can occur. Piston #6 is the most vulnerable.

Originally Posted by bigblue87

as far as the map sensor from previous knowledge i have pulled the vac line off and the engine has died.... not sure if that means anything to you.

I want the actual reading. This will help indicate if the cam can be used with a stock TBI system.

[bigblue87]

I dont have a vacuum gauge but it seems to have a good amount of vacuum.

i pulled some codes i got last time i had it out and one was for the MAP but i think thats cause i was messing with that vacuum line while it was running.

the other was i think 43... either way it was for the spark controll.