Throttle sensitive cold start 1999 Vortec 5.7

[Pinger]

Before I changed the inlet manifold gaskets it would cold start without throttle. Since then - despite winding the throttle stop screw in as far as it goes - it needs a touch of throttle pedal to cold start. This was acute yesterday and could have flattened the battery so starting to be a concern.

No pedal it offers but wont catch. A touch - and I mean the merest touch - and it will. A touch more and it wont even offer.
Yesterday, when it would catch, if I lifted off the pedal it just died. When I held the pedal in the same position the revs sat at 2000rpm (held for 15-20 seconds) and sounded 'thin'. After that it idled fine and accepted being changed over to LPG (it quit prior and wouldn't start on it either). Once warm it started - as it usually does when hot - without any pedal.

Checked the temp sender and it is reading ambient temp (cold engine) and no fault codes. Otherwise, it drives fine with nothing obvious amiss.

Any ideas as to what's going on - or not?

 

[HotWheelsBurban]

Throttle position sensor? I know the truck normally doesn't run on petrol,but this part being bad can cause lots of problems starting and staying running.
If you take it off the throttle body, the shaft should have some resistance to moving. A new one is fairly stiff. When I had to change this on my Burb, the old one(no doubt the original one) was very loose. When it would run, the idle was all over the place fluctuating.
Also, are all your battery and ground wire connections tight? If my battery cable terminal is loose (--cable) a little, it'll be reluctant to start w/o a touch of throttle pedal. Hope this helps....

 

[HotWheelsBurban]

Also could be the coolant temperature sensor in the front of the intake manifold,under the thermostat housing. This is the one that talks to the PCM and tells it how to set the mixture. I've had intermittent issues with mine, especially when colder or damp.

 

[Schurkey]

What is the fuel pressure at key-on? Does the pump prime when the key is turned to "on", but before you crank the engine?

 

[Pinger]

Throttle position sensor? I know the truck normally doesn't run on petrol,but this part being bad can cause lots of problems starting and staying running.
If you take it off the throttle body, the shaft should have some resistance to moving. A new one is fairly stiff. When I had to change this on my Burb, the old one(no doubt the original one) was very loose. When it would run, the idle was all over the place fluctuating.

Crossed my mind later that it could be that. I'll check it later today with 2 different diagnostic tools.

Also, are all your battery and ground wire connections tight? If my battery cable terminal is loose (--cable) a little, it'll be reluctant to start w/o a touch of throttle pedal. Hope this helps....

Battery ones are perfect (and it's a recent battery). Grounds are OK I think.

Also could be the coolant temperature sensor in the front of the intake manifold,under the thermostat housing. This is the one that talks to the PCM and tells it how to set the mixture. I've had intermittent issues with mine, especially when colder or damp.

That's the one I checked via the lap top - it read 15C - just a touch lower than the prevailing air temp. When I have 210F on the gauge (normal running temp), the thermostat sender is showing 198F on the diagnostics -which seems about right.
It did pour down rain the previous night - torrentially. In the back of my mind was damp in the ignition (dizzy cap) but when it ran fine after 15-20 seconds at 2000rpm I kind of dismissed that idea. Maybe the temp sensor was affected - I'll take a look for any signs of dampness around it.

What is the fuel pressure at key-on? Does the pump prime when the key is turned to "on", but before you crank the engine?

I don't have any way of measuring that.
However, it offers the moment it starts cranking - just won't 'catch' (and run) unless I give it the slightest amount of pedal. More pedal though, and it wont even offer - which confuses me. Also, I'd have expected it to start on LPG but it didn't. The LPG system uses the TPS - the only shared sensor other than O2 sensors which were too cold to be relevant.
Once warm, it's fine - starts even without a touch of pedal. The problem I'm having is the same as immediately after the inlet manifold gasket change and was cured then by winding in the (tamperproof but seal already broken) throttle stop screw. It is now as far in as it will go.
It idles well also - kind of eliminating the IAC valve as a potential culprit.

I'll check the TPS later and see what gives there - seeing as that is the most obvious place change occurs when I give it a bit of toe.

 

[Schurkey]

Set the base-idle screw back where it belongs, let the IAC control idle speed as intended, see what happens?

 

[Pinger]

Set the base-idle screw back where it belongs, let the IAC control idle speed as intended, see what happens?

It will be as it was immediately after inlet manifold gasket change - non starting. It won't idle either at anything below its current setting. Unless the IAC didn't get the chance to adjust?
I can maybe try this later but not now as I've a 100+ mile trip this week and don't want to risk messing anything up - especially idling as I'll be in a city with stop-go traffic. If you believe this will help I will try it after my trip.

Two questions in my mind though. One is why that screw has been moved before. And why this problem arrived after the gasket change. I cleaned carbon off the back of the throttle butterfly and put it down to that but not so sure now. (If carbon had been obstructing airflow I'd expect it to now want less throttle angle.)
What is the GM PCM strategy for cold starting re throttle position? I know WOT cuts the fueling to clear a flooded engine but what about part throttle? Why only slightly above the throttle position it will start at wont it even offer?

Just checked the TPS. On my LPG software it shows 0.77V at idle position (circa 4.5V at WOT) and on the Delphi software connected to the OBD port shows 0% at idle position and 100% at WOT - which is what the PCM should be seeing. Voltage seems to swing with the pedal as expected.
Nothing amiss at the coolant temp sensor and it is reading a realistic 15C (inlet air at 18C).

Also, when I tried a 'forced' LPG start it wouldn't - and I expected it to. And, the switchover to LPG wasn't as predicable. It is set for 1500 rpm on decel. That is, as the rpm drops to below 1500 rpm on a closed throttle it switches. A 'blip' to above 1500rpm accomplishes that. It should have done it on the way back from 15-20 seconds of 2000rpm but didn't.

 

[Pinger]

Set the base-idle screw back where it belongs, let the IAC control idle speed as intended, see what happens?

I've given this a bit more thought and what I could try is to wind it back with the engine already at operating temp (as opposed to cold and it refusing to start) and see if the IAC can cope.

 

[Schurkey]

Based on not-very-many samples largely culled from internet forums, I've concluded that the most-common reason for dicking with the base idle screw is that the guy doing it doesn't know any better. Once it's been tampered with, other people come along and "adjust" it some more, trying to correct the original problem that wasn't fixed when the first guy played with the base idle; and the additional problem(s) created by the incorrect throttle blade position.

Thus my suggestion to return the throttle and IAC back to "stock"; and then look into whatever problem created the desire to mess with the idle screw in the first place.

The downside to this is that, for all I know, messing with the idle screw is a suggested procedure when converting to LPG. I'm WAY behind on LPG conversions. Like Sargent Schultz, "I know NNNNNOthing!".

 

[Pinger]

I'm going to wind it back in with the engine hot and if the IAC compensates then hopefully progress. If I do it cold it will just die.

My suspicion is that in the LPG conversion it was messed with - or in a subsequent attempt at getting better running on LPG when what was needed was a service.