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Posted: Thu Jul 10, 2008 2:47 pm Post subject: runs unevenly when warmed up
I am hesitant to post concerning my problem, as I feel I have checked EVERYTHING I can think of to eliminate my engines runnability problem. My engine is completely stock 305, 4bbl, & computer controlled.
When cold and started up, put in gear and go, it runs like it should. But after it warms up goes into closed loop, it runs unevenly, like a intermitant surge or slight miss.
When I said everthing I mean just about that--even replacing the harmonic balancer. Recently, I replaced the intake gaskets, thought it might be a valley side leak. New pcv, esc module, rebuilt carb with rebushed throttle shaft, tps, baro, map, different distributor, cap, plugs, wires, egr, egr solinoid, o2 sensor, knock sensor, mixture control solinoid ohms out at 24 ohms, thermostat, the muffler was replaced, the catalytic converter was removed before I bought the vehicle, etc.
The transmission (200r4) has been rebuilt recently. There are no codes stored in the computer, or service engine light on. If the TCC is malfunctioning, could it be my problem, or is replaced during a tranny rebuild??
Last but not least, I have tried to find another wiring harness, but to no avail.
ANY SUGGESTIONS WOULD BE GREATLY APPRECIATED!!!
Pops 86 _________________ 86 El Camino SS Z15 / Posi / 305-V8-4bbl
http://elcaminocentral.com/users/pops86camino/
Have you checked the MCC dwell? It should be fixed at 30 in open loop. If it's varying when idling in closed loop the ECM is reacting to some incorrect feedback. How do you know it's in closed loop? Could it be switching in and out of closed loop? Do you have a scanner? _________________ Ernie
87 El Camino
"To be Old and Wise, you must first be Young and Stupid"
Thanks for your reply.
Yes, I do have a scanner, Actron cp 9145. It helped me to discover that the tps was not working. Yes, I have checked the duty cycle. With my dwell/tach, I have adjusted the mcc to 30 in closed loop. The scanner tells me it is in open or closed loop. I have noticed it has went to open loop when idling too long. Revving the throttle a few moments takes it back to closed loop
Sir, I did not realize the dwell should be FIXED AT 30 IN OPEN LOOP. also, you are right when idling in closed loop it does vary. I am of the understanding that in closed loop the dwell should vary as the computer through the o2 sensor adjusts the dwell time to fine tune the mixture under the current operating conditions. I will hook up the scanner when the engine is cold and warming up. The few times I have used the scanner, I have not hooked it up until the engine was warmed up.
Maybe you have hit on something I need to look at closer tomorrow, if it is indeed varying in open loop. I will find out.
... I have noticed it has went to open loop when idling too long. Revving the throttle a few moments takes it back to closed loop
That is because the oxygen sensor cooled below operating temperature. Revving the engine heats it back up. _________________ Ernie
87 El Camino
"To be Old and Wise, you must first be Young and Stupid"
I connected my scanner while the engine was cold. The dwell was fixed at 30 while in open loop. The engine warmed up to 167 degrees and went into closed loop. The dwell began to vary.
A question is: If I hold the throttle steady at say 3000 rpm shoud the dwell eventually return to 30, or will it remain at say 60?? I am trying to understand if at idle it should be at 30, then when accelerating the dwell should increase until a steady cruise speed is reached, and the dwell return to the optimum of 30 degrees?
A question on spark plug wiring routing. Originally the plug wires on this engine came off the cap down the back of the engine between the firewall, then on each bank turned forward along the pan bolts/gasket, underneath the exhaust manifolds, and then each wire connected to its respective spark plug. Now, on the left side (drivers), it is like original, the passenger side, the wires come off the cap drop down by the ac accumulator, drop down to the frame, and then connect to each plug. Could this routing cause an uneven spark (crossfire) condition?
If I hold the throttle steady at say 3000 rpm shoud the dwell eventually return to 30, or will it remain at say 60?? I am trying to understand if at idle it should be at 30, then when accelerating the dwell should increase until a steady cruise speed is reached, and the dwell return to the optimum of 30 degrees?
Let me preface my answer by first describing the theory of operation for fuel control in our CCC system.
The ECM switches the MC solenoid "ON" and "OFF" 10 times per second. The amount of time the solenoid is energized for is the dwell. At 60* dwell the solenoid would be energized 100% of the time. At 0* dwell the solenoid is de-energized 100% of the time. Consequently, a dwell reading of 30* means the solenoid is energized 50% of the time.
In closed loop operation if the carburetor and other engine systems are operating correctly the acceptable range of dwell variation is 10*-50*. An increase in dwell readings mean the ECM is commanding a leaner air/fuel mixture. A decrease means a richer misture. The goal being to keep the air/fuel mixture at a desired 14.7:1 ratio (also known as the Stoichiometric Ratio). Many sensors help the ECM determine the dwell time in closed loop. The throttle position, oxygen, manifold absolute pressure, and coolant temperature sensors all affect the dwell.
With all of this being said the answer to your question is that the optimum dwell of the MC solenoid is whatever it needs to be to maintain the 14.7:1 air/fuel ratio.
pops86 wrote:
A question on spark plug wiring routing... Now, on the left side (drivers), it is like original, the passenger side, the wires come off the cap drop down by the ac accumulator, drop down to the frame, and then connect to each plug. Could this routing cause an uneven spark (crossfire) condition?
It's possible that cracked, frayed, or burnt plug wire insullation could cause a rough idle condition. If they are in poor shape get a new set and eliminate the possibility. Make sure to get good wires and keep them away from the exhaust manifolds. _________________ Ernie
87 El Camino
"To be Old and Wise, you must first be Young and Stupid"
I apologize for the delay in getting back to answer Robby85, but I work 300 miles from my home and have to wait for the weekend to do anything on my elc.
Let me say this first in response to Robby85, initially, the mc dwell in closed loop varies from 29 to about 35. Today I checked the timing, it was nearly perfect, moved it about 1-2 degrees. Then I adjusted the idle speed which was about 650 and varying a little, but could not get it down to 500 without stalling. I noticed when it stalled the dwell time went to nearly 60 degrees, full lean. The slowest I can idle it is about 550-575, varying.
I checked the float level idling, it measures 12 to 13/32ths, If I am not mistaken it should measure 11/32ths.
What I am wondering is, as far as I know the well plugs have not been sealed. Could this be a the problem since I can see no fuel overflowing while looking down the primaries with the timing light illuminating area.
What other sources of enrichment can I look for , as the more I think about it, this is probably what is causing my uneven running condition.
Thanks for your time and ideas!!!!!!!!!! _________________ 86 El Camino SS Z15 / Posi / 305-V8-4bbl
http://elcaminocentral.com/users/pops86camino/
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