1986 305 Emissions question

I have a 1986 Silverado with the 305....

Engine is recently rebuilt, bored .030 over, and has good (160) compression in all cylinders.

Original (OEM) carb rebuilt (throttle shaft bushed as well), new plug wires, intake passages all cleared (media blasted), 3 angle valve job, and most emissions devices tested and/or replaced.
New plugs, distributor cap, rotor etc.

New catalytic converter & stock exhaust system.

Distributor has been detailed (shimmed etc).

The engine has given me emissions problems from day one since the rebuild. I didn't own the truck before the rebuild, so I do not know its history.

It has double the allowable hydrocarbons in the sniffer test.
EPA says this may be a vacuum leak.
I tried the "guaranteed to pass emissions" stuff from Advance Auto... twice... it still failed.... with fresh oil both times.

As far as I can tell, there are no vacuum leaks... but I must be missing one. I replaced & repaired many broken vacuum lines and I cannot find any more broken or cracked lines. Maybe the intake manifold is not sealed properly at the heads.

The power brakes works fine, and I cannot find any vacuum leaks at the diaphragm or in the vacuum line.

The parking brake release vacuum control does not leak.
I checked for leaks in the heater controls under the dash... no leaks.
I removed the entire dash, heater & all to verify this.

I checked for vacuum leaks in the cruise control... no leaks.

When set to factory specs, the engine does not run. It has to be set "outside the envelope" to keep it from stalling.

Question:

Is there a forum or web site where I can find information specific to the 1986 C10, in particular, emissions problems & solutions?

Thanks in advance...

what do you mean 'set outside the envelope'?

By outside the envelope... I mean the two idle mixture screws are out 3 to 4 turns instead of the average 2 to 3, and the timing has to be advanced.

I also have to mess with the APT setting based on outside temperature (100 degrees vs 30 degrees).

Otherwise, the engine runs rough and stalls intermittently.

By outside the envelope... I mean the two idle mixture screws are out 3 to 4 turns instead of the average 2 to 3, and the timing has to be advanced.

I also have to mess with the APT setting based on outside temperature (100 degrees vs 30 degrees).

Otherwise, the engine runs rough and stalls intermittently.

if you have the E4ME carb Id say the MC sols are not working, and as for the timing, if you have EST you have to set it a little differently with the EST disabled.

I have the M4ME carb... and the LE9 GM engine option. The M4ME is different than the E4ME.. The carb only has one solenoid on it which appears to be working properly. I believe it simply controls the fuel level in the float bowl based on engine temp. I also replaced a bad knock sensor and tested the ECM using the procedure in the factory service manual using an ohm meter. I bought a new ECM and compared readings... the original ECM measured the same as the new (rebuilt) ECM unit.

But... I will re-visit that solenoid just to be sure if it's a possible cause for high hydrocarbon emissions at idle.

I have the M4ME carb... and the LE9 GM engine option. The M4ME is different than the E4ME.. The carb only has one solenoid on it which appears to be working properly. I believe it simply controls the fuel level in the float bowl based on engine temp. I also replaced a bad knock sensor and tested the ECM using the procedure in the factory service manual using an ohm meter. I bought a new ECM and compared readings... the original ECM measured the same as the new (rebuilt) ECM unit.

But... I will re-visit that solenoid just to be sure if it's a possible cause for high hydrocarbon emissions at idle.

when you say ECM, you mean the computer? or you mean the TPS? also, when you rebuild a carb with a mixture control solenoid - that makes it an E4ME, unless you mean there is no solenoid and you are looking at the idle step solenoid? suffice to say, if you have to open the pilot screws more than spec, then something is interfering with fuel flow.

Yes.. the truck has a computer box up under the dash. I located it to test the anti-knock circuit as outlined in the GM service (shop) manual.

The carb is a M4MED... it does not have a mixture control solenoid. It has a solenoid to control the accelerator pump discharge volume. When the engine is below 170 degrees (cold) the pump provides a full shot. When the engine is hot, the pump discharge volume is one-half.

I re-read the manual last night regarding the "dual capacity pump" solenoid. Since the pump only provides a shot of fuel when you actually step on the gas pedal to activate the pump, I think I can assume that it would not affect a sniffer test at idle.... so I am ruling it out as a cause for my emissions problem... even if it is bad (I think it's okay) it should not affect hydrocarbons at idle with my foot off the accelerator.

I have the GM Factory Shop Manual which describes the carburetor in detail. It is not a California emissions vehicle..... definitely has the M4MED carb.... no mixture solenoid.

I just cleared a space for the truck in my garage so I can move it inside and pull the intake to replace the intake gaskets. I can't see any other options at this point.

If there were a source of information specifically geared toward emission issues with this year/engine./carb... that might save me some grief.

Have you tried hooking up a Vacuum guage?

I'll read a paragragh out of my service manual.

Read the vacuum gauge; an average, healthy engine should normally produce about 17-22" of vacuum with a fairly steady needle.

a) A low steady reading usually indicates a leaking gasket between the intake manifold and throttle body, a leaky vacuum hose, late ignition timing or incorrect cam shaft timing. ( Book is showing approx 11-12")

b) If the reading is 3 to 8" below normal and it fluctuates at that low reading, suspect an intake mainifold gasket leak at an intake port or a faulty fuel injector. (Book is showing 12-17" back and forth at the same speed as the intake valve)

I'll stop at this point, but there are 7 differant scenairios for vacumm readings. You might consider some more testing before you wrench!!

-Mike

Mike... yes... you are right... I've been using a vacuum gauge on this problem for over 2 years. The problem I am having with the tool is that there are so many vacuum operated devices on this engine, that I have not been able to isolate any particular cause/effect relationship. To top it off, each time I get near any of the factory plastic vacuum lines, they shatter and I have to make a new series of repairs. I have patches all over the place. The vacuum hoses are a "can of worms" on this engine.... check valves... in-line metered orifices... you name it.

My plan is to do exactly what you suggest before I tear the intake off... one last time. For the amount of hours I have in this little demon, I could have bought a different truck already.

At one point, I actually discussed having a Chevy Dealer find the problem and repair it.... but they told me to go away... they didn't have anyone on staff that knew how to work on a carbureted engine. They actually refused to service the truck.

At another Chevy Dealer that was doing the emissions test, they got so tired of my truck, that they let me work on it while they tested it to try to get it to pass.... it didn't.

A "normal" person would have given up by now.
(heh heh)

I have a spare Q-Jet non-electric carb off a 77 GM truck (needs a rebuild), and I have a SBC points type distributor that I can switch in for testing as well to eliminate the computer controlled HEI. But... I'm still going to get an intake gasket set before the stores close.

You do have a tuff nut to crack!

I give you alot of credit for your tenasity.

Good Luck with it, and let us know if you have a sudden revilation!!

-Mike

has anyone ever been able to solve this? Brother in law having same problem with 1986 truck with 305 in it. How do you test emission modules on the quadrajet?

I have had a few quadrajets that I just could not get to work right. Had to replace them with new. Also had problems with factory rebuilds. The new work better if you can find one.

Do the aftermarket edelbrock carbs support the emission controls? Could the problem be the electronic control module under the dash?

I do like the Edelbrocks. They have worked will on everything I have tried them on, but I have never tried them for an emission problem. I would ask Edelbrock. I know timing has to be spot on. The control unit is for advance so maybe.

Guys this thread is 3 years old,. Start a new one, please!