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Car starts better when not primed?

5K views 34 replies 10 participants last post by  eagleeye 
#1 ·
Hello all,
I have noticed that when the engine is cold the car starts much better if instead of putting the key in "on" and letting the fuel pump do it's thing before you crank, you turn straight from "off" to "start".
It starts either way, it just seems to struggle a little if you let it build pressure. It has done this as long as I can remember. It seems to make a difference in cold weather, not so much during summer. When the engine is warmed up it makes no difference.
Is this normal, and is there any harm in not priming the fuel pump?
Thanks!
 
#3 ·
I have to disagree... however, I would call it 'glowing'. We traded in a VW TDI (Diesel) which *HAD* to wait for the preheat-lamp before cranking the engine.

After 10 years of 'glowing' the VW before starting the engine, we just naturally wait for the needles to 'cycle' on the Outback.

I would have to say that we REALLY miss the 50+ MPG. The outback has never approached 800 miles per tank of fuel.
 
#5 ·
Priming only works on cars with carburetor (one that has an enrichment pump activated by wide open throttle).

Between the fuel injectors and the fuel pump check valve closing on shutdown, system pressure is maintained for days or longer. This has been the case since at least the 80s. Heck, it was probably even the case for the first mechanical fuel injection systems introduced by Benz in the 50s!

I owned a 80s car with Bosch K-Jet mechanical fuel injection, which was pretty much a direct descendent of the early Benz FI system. Having pure over the factory manuals -- I fix my own stuff -- it was pretty clear that the air flow sensor didn't work as an enrichment device. That was a function of various temperature sensors -- one in the engine block and a bi-metalic spring inside the mechanical injection pump.

Fast forward to the 90s and the Bosch LH system, which uses a hot wire (LH = "luft" or air and "heiss" or wire) to measure air temperature and an Engine Coolant Temperature to measure block temperature to establish the correct parameters for a cold start.

Todays systems, like the ones in a Subaru (Denso, probably, but I haven't had a good poke around since getting my car as winter conditions started here) work similar to LH systems. They use a slightly different method of measuring air density (manifold absolute pressure, or MAP, sensor and an Intake Air Temperature, or IAT, sensor), but the general principal of operation is the same.

Pumping the throttle before ignition does absolutely nothing because the electronic control unit cannot read the throttle position sensor. If it somehow could, the car would run like crap as the ECU would read "open throttle" and fail to properly set the injector pulse duration to provide cold start enrichment. (Most cars these days don't have an extra cold start injector to provide initial enrichment.)
 
#6 ·
I have never touched the gas pedal when or before starting the Outback. I'm referring to letting the fuel pump pressurize the line (as it makes it's hum) letting the car sit in "on" for a few seconds before I start the engine.
It definitely does something. In cold weather, with a cold engine, there is a very significant and repeatable difference between when I do let it pressurize and when I don't.
 
#8 ·
Gotcha -- I got pulled off track by the term "priming", which is a term used back in the day to describe stabbing the throttle a few times before starting.

The pump will run in the manner you describe. The hum you hear is the pump taking the system from its resting pressure to full line pressure.

Resting pressure in a fuel injected car is more than enough to start the car. -- unless there's a fault somewhere.

The bottom line here is that waiting until the pump stops cycling shouldn't result in any significant change in how the car starts -- unless the check valve in your fuel pump is bad, the fuel pressure regulator is shot, or you have an injector leak (as per Jeep667's comment). If any of these components were faulty, the car would run like crap (or not at all) and probably trigger a "check engine" light.
 
#9 ·
My driving is a city/ highway/ back road mix, probably 20/20/60%. It is also frequently used for 1/2 mile trips when my Mom uses it for shopping. On premium, it's been getting 24 in the warm months and 20 during winter.
That seems like decent fuel economy, there's no CEL, and the car has never failed emissions.
 
#12 ·
I didn't realize I had an issue.
This explains so much. The car has had a slightly weak cold start for about half it's life. Several years ago it failed to start in the morning and the dealer said the engine was flooded even though the gas pedal had not been touched. The car has also had a rough, vibrating idle when cold and in drive with a foot on the brake (axles are original OEM.)
How would I go about diagnosing and fixing a leaky injector?
 
#13 ·
The car has had a slightly weak cold start for about half it's life. Several years ago it failed to start in the morning and the dealer said the engine was flooded even though the gas pedal had not been touched. The car has also had a rough, vibrating idle when cold and in drive with a foot on the brake (axles are original OEM.)
All of these issues are strong indicators of leaking injectors.

How would I go about diagnosing and fixing a leaky injector?
A mechanic has to remove the injectors, hook them up to a test rig, apply pressure to the injectors and watch for leaks.... or

Pull the plugs. Look for the cylinders that appear to be running rich. Replace the injectors for those cylinders.

I didn't realize I had an issue.
Yup,you do. It's been going on for years its seems.

Good on Jeepj669 for leading the discussion towards the injectors!
 
#15 ·
The regulator or check valve wouldn't cause your hard start issue though. The only way to check individual injectors is by removing them and using an injector tester. A way to narrow it to injectors leaking for sure before removing them is to unplug the harness or all the individual injector plugs (haven't done on a subie yet so don't know how hard that might be) then "prime" the fuel pump 2 or 3 times then try to start the car. If it tries to fire then one or more injectors is leaking fuel into the cylinders.
 
#17 ·
The standard static pressure tests I've outlined was to teach how to test for a suspected fuel bleed down problem whether or not circumstantial evidence points to a particular component. It also verifies the integrity of a component that passes the test at the same time.
Would the following test work?
A way to narrow it to injectors leaking for sure before removing them is to unplug the harness or all the individual injector plugs...then "prime" the fuel pump 2 or 3 times then try to start the car. If it tries to fire then one or more injectors is leaking fuel into the cylinders.
If an injector(s) is leaking enough fuel to provide a combustible mixture in the less than 10 seconds it takes to cycle the key a few times, then yes, the evidence would suggest one or more leaking injectors (which would be seen as an immediate and steady drop in the static pressure test). However, if insufficient fuel has leaked in this very short time frame to provide a combustible mixture, this test would not only be inconclusive but may lead one to believe that the injectors are not leaking.
 
#18 ·
A leakdown test has confirmed that the fuel system is leaking.
I called around to try and find a shop that had a spray tester. One said that they didn't have a spray tester, but that they could pull the injectors from the engine, prime the pump to pressurize the lines, and see if anything comes out of the injectors.

Does that sound like a good test?
 
#21 ·
Because you're not just looking for leakage, but spray pattern too. A dirty or defective injector will deliver fuel that looks like a stream of p*ss. A clean, functional injector will deliver a symmetrical conical spray.

Spray pattern makes a massive difference in starting, idle quality, etc.
 
#22 ·
Here's the issue that I'm facing right now.
I want to have this fixed before a 1500 mile road trip on February 16th. I have found one local garage that has a spray tester, and they're not even really local. They're 40 minutes away. They are a fancy high end shop for Mercedes Benzes and Lamborghinis, and they said they won't work on a 2003 Subaru (which probably means I can't afford their prices.)
The dealer (Goldstein Subaru) doesn't have a spray tester.

The car is needed for day to day transportation, so parking it for a week while the injectors are on vacation isn't really an option.
 
#23 ·
Simple swap cars with the old man,...he gets AWD, you get MPG.


1500 miles in a 42mpg VW Diesel Vagon = 36 gallons

at Today's Diesel price in your neighborhood (near RPI) = $3.54 (according to gas buddy) = $127.44

vs. 1500 in the Bean at 26mpg = 68 gallons
at Today's price Premium in your neighborhood (near RPI) = $3.00 (according to gas buddy) = $204
 
#24 ·
How much are 6 new injectors? You can then get your old ones rebuilt and sell them (or sell them as is).
 
#27 ·
and on 2nd thought. Did you consider flying to out of Albany?
and if you are on a college expedition you should be looking at brainy MIT, :nerd:

...not the schools for morons :ucrazy: in Chicago or alcoholics in :7:North Carolina.
(each 750 miles out of Albany).


Concerning a long car trip vs. a short airplane flight: Make your pop listen to this (from this mornings NPR morning edition):

The Downside Of Cheaper Gas: More Accident Fatalities : NPR
 
#28 ·
I should explain the trip.
It is a family trip. We are visiting a couple colleges in Michigan, and meeting a friend there. We are also visiting a couple colleges in Indiana, including Purdue Lafayette.
While we are there, we are getting a tour of the Subaru factory.

So it's a multi college expedition/ family road trip/ car homecoming.

I might just leave the injectors alone for now. It's been fine for years with this problem (which we didn't know it had), it'll be fine for another few months.

In a few months my Mom will be buying either a new Forester or Outback. The '03 is currently shared by us and is her transportation to work. When she has a new car it will be possible for it to sit for a few weeks while the injectors are off being tested.
 
#33 ·
I was just thinking 20 (some odd) years ago i had a car that had a fuel injector stuck open. rough starting, flooding was the things that would happen until I got it replaced.

that was a 4 cylinder VW with K style CIS fuel injection,...(which came before EFI,...and rudimentary)
https://en.wikipedia.org/wiki/Jetronic

I was in high school,...85mph was as fast as the circular speedo read, ...but in this brief time I could could actually bend it way past there like 93-95mph.
 
#34 ·
Have you changed spark plugs recently? If not, consider doing so.

What shape is your battery in? Had it and your alternator tested lately?

If indeed you have a leaking injector(s) you may see benefits from initiating fuel flow prior to starting as some of your fuel isn't getting where it's supposed to be.

Do a little preventative maintenance before you tear down your injection system. Making sure your battery, grounds, alternator output and plugs are all ok before taking such a long trip should be done anyhow.
 
#35 ·
03LLBean, really (REALLY) cares for this car. ...it is the opposite of ignored.

he is a one 17 year old pit crew. hence threads like this,...where others would just drive it until it needs a flat bed.
 
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