Engine runs rough when cold

Tiny
OLD.N.BOLD
  • MEMBER
  • 1996 CHEVROLET S-10
  • 162,500 MILES
When 1st starting & running it misses, sputters when revving it up & also the tachometer doesn't work. When it gets warmed up it runs great&the tachometer will work? Does the tachometer need vacuum? Are the problems really related like they seem to be? Where do I start? Like I said, when it gets warmed up it runs great
& the tachometer works correctly.
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Monday, October 17th, 2011 AT 3:38 PM

2 Replies

Tiny
DRCRANKNWRENCH
  • EXPERT
There are few possibilites, I am providing you with a list of them. Note that the distributor pick-up coil is a possibility.
I am also giving you a partial desription of ignition system.

The reason I am puttng together the symptoms and issues is that the tach gets its signal from the coil.
If, during start-up the timing is off or part of the Idle Air Circuit Control Valve, (IACV), is faulty, the timing can be thrown off which would cause the rough starting idle and the tach signal interference.
Timing is retarded some during warm up, if there are other issues in the start up, like the camshaft position snesor or IACV, the timing is thrown even further off than when warmed up.
I am providing you with a sdiagram of IACV circuit as well so you can see al parts invlolved insytem.

So, my thoughts are;
The coil is failing
The camshaft position sensor is failing.
The ignition timing is off due to worn out timing/beltchain.
The IACV is failing and causing a series of issues during start up. The IACV also works like a choke to let the air/fuel mixture lean out during start up.

However, the tach could be gettng a faulty signal from a lower than specified voltage signal form the rough idle which is probably lower than it should be. In other words, the information I am giving you is a good back up of possile failed vcomponents. The tach malfunction makes it a little harder to diagnose but makes me think it is an igniton control and/or timing issue causing the problem.
Also, the fuel pressure can cause rough idle as well as vacuum leaks.

The first thin to do is go to an Auto Zone or Avance auto parts, etc. And have them pull troubl ecodes. You do not have to have a malfunction light on to have a code. It will help in the process of eleimination of the source of the problem tremendously.

Let me know if you need any more help with this issue and I will reply ASAP.
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Monday, October 17th, 2011 AT 5:59 PM
Tiny
OLD.N.BOLD
  • MEMBER
I am trying to reply to DrCranknWrench about his answer to my.
I took my '96 S10 to Advance&had it scanned. It showed low voltage
with the MASS. But when I started it to go there it did not act up
at all except the tach never worked even on way back home.I'd
really like to fix the problems so please try to steer me the way
to start at.I know there always is a process of illimination but
don't want to spend a bunch of money. Thanx.
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Wednesday, October 19th, 2011 AT 2:01 PM

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