S-10 Rough Idle

[Keith Tedford]

We have a 1987 S-10 truck with the automatic transmission and the 2.8 engine. It has started to run very rich of cold weather start up. Any suggestions as to what is wrong would be appreciated. Not too computer literate. Just a carb guy.

[pappyl]

could be more than a couple things

Running Rich
- most probably your coolant sensor (in the intake)
- may also be - map sensor (mounted to your breather) - see if you get black smoke coming out from your tailpipe on acceleration
- may also be - your O2 sensor - if it's old - replace it - save the money back in gas


Rough Idle
- IAC on your TBI may be plugged or going.
- Sparkplugs/distributor/rotor
- coil
- ignition module - base of distributor

[Keith Tedford]

The truck runs very rich like an over choked carb when first started. When revved up it clears and runs fine. Only at startup is there a problem.

[BlazerLT]

Originally Posted by Keith Tedford

We have a 1987 S-10 truck with the automatic transmission and the 2.8 engine. It has started to run very rich of cold weather start up. Any suggestions as to what is wrong would be appreciated. Not too computer literate. Just a carb guy.

1.) Do a full tuneup.

2.) Get your timing set to zero with the black and brown timing wire disconnected from the PCM under the glove box. It will be under the edge of the carpet.

Don't start replacing sensors unless if you have a check engine light on. Then you can get the codes adn pin point the problem.

[muddog321]

Is this carb or TBI - if a carb check the choke cause that was a common problem on them sticking shut and nearly flooding out tthe engine on 2.8 - spray clean the carb outside and inside (use a toothbrush inside and make sure all parts move freely).

[BlazerLT]

They stopped making carbs for this engine in 1986 I believe.

[pappyl]

Could still be the O2 sensor.

It's pretty hard to nail down the problem because it could be so many things. Starting with a tune-up and take of the TBI and spray it down too.

If it starts rough, you hit the gas a couple times, and it straightens out after, it could also be the TPS sensor on the TBI. BUT you should be getting a "check engine" light on that one. Does your "check engine" light up on pre-start (ie: when you turn key to start engine)?

[pappyl]

Oh missed that previous post.


The 2.8 should be 10 degrees, not zero (check your sticker on the fan hood).
And the O2 sensor can malfunction and NOT trigger the lamp.

[Keith Tedford]

Originally Posted by BlazerLT

They stopped making carbs for this engine in 1986 I believe.

In the garage the injected truck starts normally. It if is outside it will start and idle quite fast. With temperatures at 25 degrees F. It starts rich but will clear and run fine once revved up. No check engine light. I'll start a process of elimination in a couple of days. Just wanted a most likely place to start. It seems to be temperature related though.

[BlazerLT]

Originally Posted by Keith Tedford

In the garage the injected truck starts normally. It if is outside it will start and idle quite fast. With temperatures at 25 degrees F. It starts rich but will clear and run fine once revved up. No check engine light. I'll start a process of elimination in a couple of days. Just wanted a most likely place to start. It seems to be temperature related though.

An engine will always run faster when cold to warm up, this is normal.

Remember, all engines run rich at startup as well, it is so the engine warms up faster.

Don't rev the engine when it is cold, it will only cause wear damage.

[Keith Tedford]

Originally Posted by BlazerLT

An engine will always run faster when cold to warm up, this is normal.

Remember, all engines run rich at startup as well, it is so the engine warms up faster.

Don't rev the engine when it is cold, it will only cause wear damage.

This one runs excessively fast for a short time then settles down. None of our injected vehicles have ever run excessively rich at startup. This has just started lately so I'm thinking that it is a sensor problem. Once running the truck runs fine in all other ways.

[pappyl]

Couple thoughts for ya on this...

You'll get a "too fast idle", especially during startup, when...
1) you have a vacuum leak - few lines running to egr, cruise, thermac, map sensor, and air pump (i am beginning to suspect this is the culprit), TBI gasket leak
2) you're timing is advanced too far

Please note....
When your engine is cold it is running on "open-loop" mode vs. "closed-loop" meaning the feedback from the sensors are not being used to control the fuel or timing - it's following a set pattern.
Once the ECM sees the temp up to running - it goes to "closed-loop" using the map, o2 sensors to control the motor.

[BlazerLT]

Originally Posted by Keith Tedford

This one runs excessively fast for a short time then settles down. None of our injected vehicles have ever run excessively rich at startup. This has just started lately so I'm thinking that it is a sensor problem. Once running the truck runs fine in all other ways.

So the engine is not running rough, it is just running at a higher rpm when you start it?

Don't judge how this engine will start by how the other engines start up when cold.

You are saying it has a higher rpm when colder which is normal.

Have you ever done a tuneup?

[Keith Tedford]

Originally Posted by BlazerLT

So the engine is not running rough, it is just running at a higher rpm when you start it?

Don't judge how this engine will start by how the other engines start up when cold.

You are saying it has a higher rpm when colder which is normal.

Have you ever done a tuneup?

When cold below freezing, the engine will start rich, when started around 40 degrees it starts ok but idles excessively fast. When started in the warm garage it is fine. I'm going to check all the vacuum lines as they are pretty old. I like to check the simple stuff first, just haven't had the time. Thanks for the tips.

[BlazerLT]

Originally Posted by Keith Tedford

When cold below freezing, the engine will start rich, when started around 40 degrees it starts ok but idles excessively fast. When started in the warm garage it is fine. I'm going to check all the vacuum lines as they are pretty old. I like to check the simple stuff first, just haven't had the time. Thanks for the tips.

How do you know it is running rich?