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Car hard to start when the engine is hot

dmj

Well-known member
Joined
Feb 2, 2008
Messages
673
Location
orlando FL
The car starts easily enough when it is cold ,once it is driven even for a few minutes and the car is up to operating temps like on a run to the grocery etc I have to cycle the key several times for it to start If I open the throttle a little it helps and the car starts on the second or third try. Note I did remove the egr the thermo switch, all four vac lines from the throttle body and some sort of solonoid switch on the firewall recently just to clean up the engine bay.I need help This is my daily driver .Thanks in advance Dexter
 

toybreaker

iconoclast
Joined
Apr 30, 2006
Messages
3,581
Have you checked your fuel pressure lately?

Problems with a hot restart can often be related to fuel pressure.

One possibility is that you're bleeding fuel pressure off at shutdown thru the fuel pressure regulator, injectors or the fuel pump check valve.

If the problem first showed up after the emissions delete, you may have found the reason the factory installed the fuel pressure solenoid. (One of the solenoids you removed was the fuel pressure solenoid.)

The ecu activates this solenoid on a hot restart to increase the fuel pressure, which helps hot restarts considerably.

Check your fuel pressure, and go from there.
 

dmj

Well-known member
Joined
Feb 2, 2008
Messages
673
Location
orlando FL
36psi of fuel pressure with key on. I have a fuel pressure gauge on the fuel rail.I traced the vac line from the fpr back to a solenoid on the factory air can which is still mounted on the car. Am I right in assuming this is the fpr solenoid and not the one I removed from the firewall next to the cruise control box? Note when the car sits overnight it starts at the first twist of the key so that eliminates fuel pressure as a variable. Toybreaker you may be right in suggesting the pressure solenoid. The question is how do I test it?
 

toybreaker

iconoclast
Joined
Apr 30, 2006
Messages
3,581
The actuation/functionality of the fpr solenoid should be visible on your fuel pressure gauge.

The fpr solenoid is commanded by the ecu and affects the fuel pressure by venting the vaccuum signal to the fuel pressure regulator coming from the intake manifold to atmosphere. It only operates during hot starts.

It's a little confusing, but if you think it through, you'll see how it works.



The fuel pressure regulator uses intake manifold pressure to vary the fuel pressure to match engine operating conditions.


At low manifold pressures, you get lower fuel pressures.



At higher manifold pressures, you'll get higher fuel pressures.


When you're cranking the engine over on the starter, (with the throttle closed) you'll be drawing a vaccuum inside the intake manifold.

The fpr will decrease the fuel pressure in this situation.

Those wily engineers figured out that increasing the fuel pressure during the crank mode with a hot engine would help hot restarts.

They achieved this by using the ecu to command the fpr solenoid to vent the fpr reference signal from the intake to atmosphere on hot restarts.

This means instead of the actual vaccuum in the intake, the fpr will "see" one atmosphere, (ambient barometric pressure) and that will increase the fuel pressure ~5 psi (ish).

This is the same fuel pressure you would have with the vaccuum hose to the intake removed from the fpr, and the engine idling... or key on/engine off ... as both of those situations have the fpr "seeing" atmospheric pressure.




If you eyeball your running fuel pressure with the engine hot and then shut the engine down for a few minutes, apon cranking/restart it should read ~5psi higher than it it did idling.

After a minute or two, the ecu will release the solenoid, the true vaccum signal from the intake will resume at the fpr and your fuel pressure will drop back to ~36psi.

Hope that helps, I'm not much good at explaining things like this /ubbthreads/images/graemlins/blush.gif
 

ktmrider

Well-known member
Joined
Sep 10, 2007
Messages
3,128
Location
Tempe, AZ
Quoting dmj:
I traced the vac line from the fpr back to a solenoid on the factory air can which is still mounted on the car. Am I right in assuming this is the fpr solenoid and not the one I removed from the firewall next to the cruise control box? Note when the car sits overnight it starts at the first twist of the key so that eliminates fuel pressure as a variable. Toybreaker you may be right in suggesting the pressure solenoid. The question is how do I test it?

Hmm, the one on the air can is the boost control solenoid ( and does not see vacuum ). The one on the firewall by the CC unit is the FPR solenoid. Test whether it gets vacuum at idle on the current circuit using TB's guide.

This could be the reason you are having issues. Hook the FPR source to a dedicated line on the intake manifold and see what happens.
 

Let's not get stuck on the fpr solenoid only. A bad coolant temp sensor and a stuck isc can both cause this. Key point: cracking the throttle helps it start sooner.
 
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