hey guys my 7.3 idi everytime I go to cold start it starts up then acts like it wants to die but doesn't. Then it revs back up and idles about 1200rpm then kinda start to drop. But if you shut it off then restart it then it fires up and press the throttle down and it goes to 900 for high idle and is fine. I know I have air getting into my fuel system somewhere but would that cause the problems with the high idle?
Cold advance question
- Thread starter Fordfan90
- Start date
Air in the system can do all sorts of things. But yeah that can certainly cause thjs.
icanfixall
Official GMM hand model
Our cold fast idle solenoid does not have the strength to open the throttle so we have to press down on the throttle pedal to set this when the ignition is switched on.
chumwon
Registered User
I'd be willing to bet it's the air in the lines. The main return line to the tank is known for problems. And yes cold advance takes a pump of the throttle to activate. My idle only goes up a few hundred rpms to maybe 800-850 instead of 650, I'm not sure if that's adjustable or not.
I figure I'll take a look at it today since Work is slow right now.
IDIBRONCO
IDIBRONCO
Chumwon, yes it is adjustable. Take a 1/4" open wrench and use it to turn the hex shaped end of the rod that touches the throttle on the injector pump. You can screw it in or out this way.
chumwon
Registered User
I wondered if that's how it worked but never bothered to mess with it. Thank ya Bronco, I'll be adjusting that up a bit now!
icanfixall
Official GMM hand model
There is another way to adjust this solenoid. Just loosen the two 7/16 hex bolts that holds the bracket to the pump and push the bracket towards the throttle.
IDIBRONCO
IDIBRONCO
I didn't mention that because he only needs to raise his idle by a few rpm. It always seems like it moves a lot more when I've tried to loosen the bolts to adjust it.
I don't know that much about the insides of the injection pump, but I read that they use the fuel pressure inside the pump to work the timing advance inside the pump. If the fuel pressure is not consistent coming into the pump, then it affects the timing advance of the pump and it can cause the engine to surge and rpms to vary up and down.
Macrobb
Full Access Member
You're right, but not for the reason you expect.
The stock lift pump only outputs a max of 7 PSI; I'm seeing 5 at idle. Even going down to 0(which the pump is rated for - gravity feed), that's only a 5 psi difference.
If the lift pump fails entirely, the engine will still run just fine, with perhaps a couple of degrees less advance - remember, the IP has a built in "transfer pump" that will pull fuel in and boost it up to a higher PSI for internal use.
However, if you have air coming in with the fuel... air is compressible. It does weird stuff to a hydraulic system like our pumps. Also, if the air gets in the injection lines and goes to the injectors, you get a second whammy, because the RPMs start to drop and surge due to, well, less fuel being injected.
The IP compensates by adding more fuel(more metering valve travel), and then it surges ahead because the air bubble was just a bubble, and now it's injecting more fuel.
The IP compensates, and the process repeats.
Also, I've noticed that IPs are more 'sensitive' governor wise when the cold advance is on and there is no housing pressure built up. Not entirely sure why this is, but I'm sure it has to do with pressure differentials somewhere.
