Hard start...poor idle...some surging at idle when cold. Will hardly start when temps below 40F.

Started just fine all last winter...even without fast idle when cold.

After I get going it runs like a top...but the idle is still iffy. Sometimes staying higher around 1200...other times getting below 800...running rough.

note: this is not the carbed engine

Still have this problem and am slowly going thru everything to find the cause. Ignit parts all new...cap...rotor...wires...plugs...

Suspect ignition because even if I use starting fluid...it still won't start very well below 40F. Then when it does start it seems to be somewhat flooded.

Have ruled out:

* EGR leak...removed
* vac leak...all plugged
* coils test out pretty close...but both slightly out of spec
* resistance for cap...rotor...OK
* took slotted ring out...cleaned up dust etc...and diodes
* removed and tested TPS
* have checked exhaust side plugs...OK

Any ideas????

Couple things come to mind...

Coolant temp sensor
Air Flow Meter

I take it you have disconnected the wax fast idle cam? There's a reason those are there, you know...

Scott

quote:

Originally posted by D21:
..... Coolant temp sensor ....

BINGO! My 86.5 Z24 did the same thing two weeks ago. The temp sensor had become disconnected.

It is located on the passenger side of the intake. There are two temp sensors. Single wire for the gauge and a two-wire for the ECU. Check that one.

With the key on, you should get 5V from one of the two wires in the connector. The other is a signal return to the ECU, I think.

Resistance will vary across the sensor depending on temp, but if I recall correctly, it is around 9 ohms when cold.

Problem is...my truck (early 720 '86 TBI)does not have a coolant temp sensor. I have the '86 1/2 D21 FSM covering the TBI system...and where this is supposed to be there is a plug in the alum manifold. Looks like OEM and looks like its been there since new. Can't find a couple of loose wires in that area either.

It does have the sending unit for the temp gauge on the block behind the oil filter. Maybe the ECU uses this signal?

The TBI stuff all seems to be identical as with the '86 later half D21...except I have no AIV and no coolant temp sensor.

Could be that in this transition TBI...there was no coolant temp sensor...the ECU just runs off the other sensors?

Going to test and/or remove the AFM next I guess. Still need to test most of the circuits going to the ECU for most things though...TPS included.

Found that when it cycles down to the 800 RPM level and gets rough...spraying a little starting fluid in it cleans the idle up right now...so I'm thinking it's a fuel issue...not ignit.

The wax fast idle thing was not working before I started messing with it...last winter it would start in any weather and idle just fine at around 800 RPM. But this can't really be the issue.

I can start it when below 40F...just use starting fluid...give it a little time to evaporate...seems to help if the throttle is held above the idle area. In fact it runs fine anywhere above maybe 1200-1400 RPM. I'd think TPS...but it tests out OK...not sure the signal makes it to the ECU though.

Oh, well. I never had any luck with BINGO.

Any vacuum leak will cause inadequate fuel pressure. Check for good, strong manifold vacuum at the FP regulator on the back of the TB.

I diagnosed mine with a hand vacuum pump hooked to the regulator. When I put the vacuum to it, the idle cleared right up.

Better yet would be to connect a fuel pressure gauge to see what you are getting.

well dont the tbi have a vacum activated choke in the airway?

quote:

Originally posted by Reserector:
Oh, well. I never had any luck with BINGO.

Any vacuum leak will cause inadequate fuel pressure. Check for good, strong manifold vacuum at the FP regulator on the back of the TB.

I diagnosed mine with a hand vacuum pump hooked to the regulator. When I put the vacuum to it, the idle cleared right up.

Better yet would be to connect a fuel pressure gauge to see what you are getting.

Good ideas. The FP regulator has a vac line running to a port at the top of the TB right below where the filter sits...so it is above the throttle plate. The fuel pressure was one of the next things on the list.

I get good manifold vacuum in the 20s, but the revs need to be up above idle...at idle the reading is a lot lower. Cannot find a vac leak...have everything off and plugged, including the EGR. Seems though that this vac difference would indicate a leak?

Sprayed WD40 all around the outside of the TB while running to try to find a leak...no difference.

This TBI has a fast idle solenoid for the the AC (not working)...but no butterfly is used for a choke. Could be some kind of internal vac leak related to this fast idle solenoid?

My FP vacuum line ran to the top of the TB also. I moved it to the manifold and it runs like a champ. Give that a try.

I tried putting about 5" HG to the FP reg and it made the idle get worse....wouldn't you know.

Seems strange that it would run just fine above 1200-1400 rpm if it was a fuel pressure issue. Going to buy a pressure tester and see where it's running at...might be the regulator is fuxored.

Tested the FICD...it works and increases the idle some when actuated and adjusted...but doesn't help the idling all that much.

Need to run all the continuity and voltage tests for everything yet.

I had a similar condition that plagued me for months. It turned out to be a broken wire in the harness. In my case it was the shielded signal wire from the TPS back to the ECU. It broke inside the insulation right at the point where the ground braid was trimmed.

I recall reading good feedback on that wire while testing. I just did not know at the time that the "good signal" was going nowhere.

One symptom I had was that it was drivable as long as I held my foot on the clutch pedal just enough to activate the switch.

Found the CTS and the sending unit for the temp gauge down under the thermostat housing.

Will install a new CTS...might be the problem.

Hopefully that's it. Check my earlier post on how to test. It is in the FSM, also.

CTS was not the problem.

The air flow meter has tested to be out of spec...puts out 1.5 to 2.5 V instead of 0 to 5 V. One on the way. Sure hope this fixes it as I'm running out of options...unless I install a weber.

The only wire I found that wasn't like the 86 D21 TBI is the output wire to the ECU from the TPS...D21 is supposed to be white and going to a certain pin...720 TBI doesn't have a wire at this pin. So looks like the ECU is a little different between the 720/D21 for '86?