1987 Chevy Misfiring when cold

[ericb]

I have a 1987 350 CID Chevy Silverado 1/2 ton that misfires something terrible when the motor is cold (approximately 50 degrees Fahrenheit and lower).  This misfiring only happens when the connector used to set the 'base timing' is plugged together.  If you unplug the connector used to set the 'base timing', then the truck idles and runs just fine.  With the 'base timing' connector plugged together, the motor will misfire until it has warmed up.  After it is warmed up it will idle and run fine. The colder the ambient air temperature, the longer the motor misfires until is warmed up.  At 50 degrees Fahrenheit, I would estimate it misfires for approximately 45 seconds to a minute until it runs fine.  My mechanic says he plugged the truck into his diagnostic equipment and says, during this misfiring event, he can see the timing jumping all around, as if the computer is trying to catch up to or retard the timing.  This truck has two gasoline tanks. This misfiring event occurs regardless of which tank I runoff of.  In addition, this misfiring event also occurs regardless of whether the coolant temperature sensor connector on front of intake is connected or not.  It is not making the 'check engine light' come on, throw any codes, and the check engine light circuit does appear to be working properly.  The fuel injector spray patterns appear appropriate and good.  I believe this is somehow associated with the open/closed circuit situation? So far I have done the following without any progress:

a)  set base timing to 0 degrees TDC,
b)  replace plugs, wires, cap, and rotor,
c)  cleaned every single ground I could find exceptionally well,
d)  replaced ignition control module,
e)  replaced water temperature sensor on front of intake manifold,
f)  cleaned out all carbon from EGR area under EGR valve,
g)  replaced EGR valve,
h)  disconnected EGR valve during testing procedures,
i)  tested EGR valve solenoid (appears to be functioning properly),
j)   replaced IAC motor/solenoid and cleaned out all carbon from and around the valve seat,
k)  temporarily replaced the computer and prom form another truck that was running fine,
l)  tested continuity/resistance of every wire from the computer to its final destination,
m)  tested MAP sensor (appears to be functioning properly),
n)  replaced all vacuum lines,
o)  sprayed throttle body cleaner all around intake/throttle body looking for vacuum leaks,
p)  replaced fuel filter,
q)  ran a can of seafoam through the system.

Any suggestions would be appreciated.  Thanks-EricB

[Irish_Alley]

well im not going to suggest a part cause looks like you replaced most of them lol. but whats your fuel pressure at when its acting up?

[ericb]

I don't believe it is a fuel pressure problem as, if I understand correctly, fuel pressure is constant regardless of if the motor is operating in closed or open circuit, idle, high rpm's, etc.

[bd]

Correct fuel pressure is fundamental to TBI operation.  Verify that the fuel pressure is stable and in the range of 9 - 13 PSI from cold start-up to operating temperature (11 PSI is typical).  How does the fuel pressure respond when you quickly rap the accelerator?  Does the SES light illuminate with ignition and then go out once the engine is running?  Will it produce Code 12 with ignition on, engine not running?  What spark plug gap did you use?  Is there any exhaust smoke or odor?  How many miles are on the timing chain?

Erratic timing in itself is no cause for alarm.  At any given moment timing can be anywhere within a multidimensional array of possibilities, based on instantaneous input from multiple sources.

What does a scan tool indicate if you monitor engine temperature during the period the symptoms occur?  Does unplugging the CTS set a code?  What happens if you disconnect the knock sensor?  What are the integrator and block learn parameters during and after the event?  TBI trucks typically remain in open loop at idle even when fully warmed up.  Does scan indicate the symptoms disappear only and coincident with entering closed loop?

[Irish_Alley]

Yeah, not saying its the problem. but you dont know whats the problem till you eliminated the impossible

[ericb]

Guys-The SES light does light up and go out with the engine running.  The SES light does produce code 12 with the ignition on/engine not running.  The SES system appears to be working properly.  I used AC delco plugs gapped at chevy manufacturer recommended gap (I forget what that was at this point-35 thousands?). 

No smoke or odor from exhaust pipes.  I would guess the timing chain is the original with approximately 125,000 miles on it.  I've considered the timing chain as a potential culprit.

I don't have a scan tool myself so I can't answer questions related to it.  I will try and disconnect the knock sensor, disconnect the coolant temperature sensor (each at  separate time) and see what happens.  If I can find the time this weekend I will put a fuel pressure gauge immediately after the fuel filter.  There is not enough room between the firewall and throttle body for me to get a gauge in immediately prior to the throttle body.  Thanks for the help. I will report back.

[bd]

FYI - A scan tool could remove a lot of the mystery from diagnosing this problem, regardless of whether it is caused by the engine management system (EMS).  Procure an OBD-I scan tool such as AutoXray or Actron (it's a handy tool to have) or software and cable to use a laptop.

To check timing chain wear, remove the distributor cap and bar the engine over in one direction to align the timing tabs.  While watching the distributor rotor, bar the engine in the opposite direction (by hand) until the rotor just barely begins to move.  The distance the timing mark moved at the balancer is the amount of slop in the chain.  Conservatively, if slop is ~0.4" or more (8+ deg) the timing chain has outlived its service life.

[78BIG-TEN]

I,m with bd check knock sensor connection.Also I,m assuming ground is at t\stat housing,if so did you remove the bolt\stud and check for corrosion,and maybe a 0-2 glitching in and out.Had one do that causing the torque convertor to go in and out of applied rapidly ,causing it to chatter.felt like a miss-fire.Aslo had a truck with a loose trigger wheel in the distributor causing problems,but temp didnt matter.A scan tool with event logging could really be helpful here

[LTZ C20]

Like BD said, check timing chain slop. That was one of the primary reasons behind the death of my old 350. It had slop, once it was out and the timing cover was off, the chain had about an inch to 1 & 1/2 inches.

[87 Wrangler]

My tbi 350 doing the same thing, 328000km on it. More prevalent when it's cold. Iv found cleaning the cap and rotor regularly to help, but looks like you went one step further and replaced all that.
Is you heat riser duct connected to the air cleaner? Fuel filter clean/new?

Does the truck bog and shift hard if you take off right away? Iv had these issues before and it was a vacuum leak, intake rail had eaten away on the flange portion, also changed the pickup coil at the same time and presto she ran like a champ!
 

[rich weyand]

Check the last post in this thread on another forum.
www.fullsizechevy.com/forum/general-discussion/c10-forum/476084-87-chevy-tbi-350-idling-issues-new-post.html

[Irish_Alley]

not to draw attention away from what anyone else said. but did you check the fuel pressure

[ericb]

Guys-As suggested I did disconnect the knock sensor. Disconnecting the knock sensor did not make a difference.  If it matters, this also did not pop up a SES code.  I also unplugged the coolant temperature sensor. This did create a code 15 (low coolant temperature) on the SES system.  After disconnecting the positive battery cable for about 5 minutes, reconnected the coolant temperature sensor, and started the engine back up, I received no further SES codes (besides the standard #12).

Heat riser is connected.  Fuel filter is brand new as stated in original post. There is an excellent ground at the bolt that connects the alternator bracket to the intake manifold.  I have cleaned this ground location extremely well.  This is where the black ground in the wiring system which is located atop the intake manifold, which I'm assuming associated with the coolant temperature system, is secured.

I haven't checked fuel pressure yet.  I will try and take a look at the timing chain slop situation.  If this was a timing chain issue, wouldn't the motor have the same symptoms regardless of whether the connector for setting the baseline timing is plugged together or not?

Thanks for the help! I will report back.

[roundhouse]

You try unplugging the O2 sensor ?

[rich weyand]

Did you check that vacuum line from the pointer I posted above?

Is the ground strap in place from the passenger side head to the firewall?