Cold Start Vacuum and Fuel Pressure
12-14-2009, 02:11 AM
#1
Thread Starter
Join Date: Mar 2005
Location: Malvern, Worcestershire
Posts: 294
Likes: 0
Received 0 Likes
on
0 Posts
Cold Start Vacuum and Fuel Pressure
I have been having cold start issues for some time and have started many threads trying to resolve it.
Finally, today i discovered something promising. Having just fitted a Fuel Pressure Gauge, i noticed the following:
My Cold Start Fuel pressure was 50PSI. The idle hunting occurs on cold start, accompanied by the strong smell of unburned fuel.
As soon as i blip the throttle, the smell of fuel intensifies from the exhaust, and the fuel pressure drops to 40PSI and the engine runs smoothly. The Engine will run fine for the rest of the day at around 40PSI
My Cold Start Vacuum pressure is 16in/HG and when warm it is 22in/HG.
I am wondering if the vacuum pressure seems too low, and is not enough pressure for the FPR to do its job?
I have already tried switching the FPR and Injectors recently.
Finally, today i discovered something promising. Having just fitted a Fuel Pressure Gauge, i noticed the following:
My Cold Start Fuel pressure was 50PSI. The idle hunting occurs on cold start, accompanied by the strong smell of unburned fuel.
As soon as i blip the throttle, the smell of fuel intensifies from the exhaust, and the fuel pressure drops to 40PSI and the engine runs smoothly. The Engine will run fine for the rest of the day at around 40PSI
My Cold Start Vacuum pressure is 16in/HG and when warm it is 22in/HG.
I am wondering if the vacuum pressure seems too low, and is not enough pressure for the FPR to do its job?
I have already tried switching the FPR and Injectors recently.
12-14-2009, 06:04 AM
#3
Thread Starter
Join Date: Mar 2005
Location: Malvern, Worcestershire
Posts: 294
Likes: 0
Received 0 Likes
on
0 Posts
I've cleaned and replaced the IACV with no effect.
The cold start vacuum pressure is i'm sure too ow at around 16ins/HG and it gradually climbs as the engine warms (to around 22ins/HG). I'm sure this must be linked to the Fuel pressure being too high on cold starts and causing the rich running and idle hunting.
I'm just wondering what could cause low vacuum on cold start. A gap which seals as the materials expand perhaps?
So far, i have sprayed so much carb cleaner over all the potential leak points and to no avail.
The cold start vacuum pressure is i'm sure too ow at around 16ins/HG and it gradually climbs as the engine warms (to around 22ins/HG). I'm sure this must be linked to the Fuel pressure being too high on cold starts and causing the rich running and idle hunting.
I'm just wondering what could cause low vacuum on cold start. A gap which seals as the materials expand perhaps?
So far, i have sprayed so much carb cleaner over all the potential leak points and to no avail.
12-15-2009, 02:19 AM
#7
Thread Starter
Join Date: Mar 2005
Location: Malvern, Worcestershire
Posts: 294
Likes: 0
Received 0 Likes
on
0 Posts
My o2 readings are normal. I replaced the o2 sensor the other day just to make sure there wasn't an issue with it. This cold start problem occurs before the o2 sensor comes on.
Well this is an interesting development this morning. This morning, when the problem happened, i had my vac gauge already rigged up and my fuel pressure gauge.
The vac gauge fluctuated violently between 5 and 10 ins/hg. It did this as the revs fluctuated violently and at the same time, the fuel pressure fluctuated violently between 40psi and 50psi.
It makes sense that the vac pressure is fluctuating, causing the FPR to cut off and on repeatedly, causing the engine to fluctuate from rich to normal fuelling constantly.
The totally confusing bit is " how could a vacuum leak disappear as the engine heats up.
The only conclusions i can come to are:
1. A crack in the inlet manifold which expands as it warms (this seems unlikely because surely the problem would always occur when the metal was cool.
2. A join which again seals as the materials swell.
3. Some part of the evap system (maybe the purge solenoid) stealing vacuum pressure when the engine is cold. ( i have not idea about how the evap system works, or if this is possible).
5. The IAC valve is to blame.
It is worth noting that:
When the throttle is blipped hard and the engine is in this state, the vac pressure & fuel pressure stabilise and the engine returns to smooth running. This is accompanied by the strong smell of unburnt fuel.
Well this is an interesting development this morning. This morning, when the problem happened, i had my vac gauge already rigged up and my fuel pressure gauge.
The vac gauge fluctuated violently between 5 and 10 ins/hg. It did this as the revs fluctuated violently and at the same time, the fuel pressure fluctuated violently between 40psi and 50psi.
It makes sense that the vac pressure is fluctuating, causing the FPR to cut off and on repeatedly, causing the engine to fluctuate from rich to normal fuelling constantly.
The totally confusing bit is " how could a vacuum leak disappear as the engine heats up.
The only conclusions i can come to are:
1. A crack in the inlet manifold which expands as it warms (this seems unlikely because surely the problem would always occur when the metal was cool.
2. A join which again seals as the materials swell.
3. Some part of the evap system (maybe the purge solenoid) stealing vacuum pressure when the engine is cold. ( i have not idea about how the evap system works, or if this is possible).
5. The IAC valve is to blame.
It is worth noting that:
When the throttle is blipped hard and the engine is in this state, the vac pressure & fuel pressure stabilise and the engine returns to smooth running. This is accompanied by the strong smell of unburnt fuel.
Thread
Thread Starter
Forum
Replies
Last Post