Rough idle for first ~2 minutes, reving is OK, only in [super] cold weather

[jc179]

Hi All

 

Have a v6 99 Mustang, with an issue when its *very* cold (-10 c), when the car is first started misses,stumbles and shakes when idling.

If I drive or rev it beyond about 2000 rpm, its ok, and if I give it the beans it doesn't miss either... always after about ~2 minutes its fine, even if I start it and let it idle [it will smooth out]. If I restart the car its fine, but after about 4 hours of sitting in the cold, the same issue happens again. The only CEL I get is mis-fire on cylinders 2,3,4, etc seems random and not always caught.

 

I've changed

- air filter

- fuel filter

- spark plugs

- wires

- coil pack

 

Car has 500,000 km on it, so lots could be worn out , I suspect the injectors are probably sticking in the cold, but want to see what else could be the issue before I go and replace them.

 

Thanks,

Jonathan

[9838stang]

When is the last time the fuel filter was replaced? Just curious

[jc179]

I suppose it could be a vac leak, though, would I eventually get a code?..., or notice some other odd behavior, maybe high idle?

 

I did the work mentioned in the post this summer, this issue has actually been happening [for a few winters now], but getting a little worse every year.

 

Fuel filter was done this summer along with air, plugs, wires, coil module, and rear O2's, not that those would have any play until the car enters closed loop anyways.

 

Jonathan

[Blackmage]

almost sounds like fuel related to me, fuel pressure regulator maybe?  

[jc179]

Hmm, that's an idea, fuel pressure regulator is on the fuel rail ... I think? I've can see what looks like a "sensor" on the drivers side front part of the fuel rail, or would that be indeed the regulator? Usually regulators don't have wires coming out of them, this has 3.... Im either case I can't read the fuel rail pressure in something like "torque" ... does this work for anyone else or isn't supported on these ECU's ?

[Blackmage]

best bet would to read fuel pressure with a secondary tool that plugs into the valve on the fuel rail

[jc179]

Yeah, is there a schrader valve handily located there?

 

When its not so cold out I'll check the pressure per Haynes manual... I suspect about 43 psi or so is about right, and the rail pressure should increase when VAC is removed [i think ].

[Det_Riot]

Yeah, is there a schrader valve handily located there?

When its not so cold out I'll check the pressure per Haynes manual... I suspect about 43 psi or so is about right, and the rail pressure should increase when VAC is removed [i think ].

From what I've always been told with 5.0'sis that 39psi with vacuum line not attached is ideal. The Schrader valve is usually towards the front passenger side if the fuel rail

[410sn95]

Just FYI. 99 v6 is a return less system.  There is no Fuel pressure regulator.  Computer controls fuel pressure via pump.

[Blackmage]

Just FYI. 99 v6 is a return less system. There is no Fuel pressure regulator. Computer controls fuel pressure via pump.

Ahh nvm my comment then

Sent from my iPhablet

[jc179]

Hi

 

I found a way to measure the fuel rail pressure on these return less setups. There’s an application called “forscan” which can read the voltage and kpa/psi of the sensor on the fuel rail / chart it while driving, etc. The “demo” version only allows you to plot one, which is sufficient, but I bought the app to plot multiple. Too bad torque doesn’t have this, maybe I’ll snoop the pid and add it in if I get time.

 

When driving/idle the fuel pressure is at a pretty even 39-41 psi while driving, and doesn’t seem to really move much. I can see the FP duty cycle ranges from 25%->40% tops, that seems to all be working properly. I don’t have any top end loss / hesitation, just this cold start issue.

 

The Haynes manual I have doesn’t cover this setup, so anyone else out there able to install fosSCAN demo version and see what your fuel rail pressure is?

 

ALSO, as it’s gotten colder and I’ve been poking about, the car now will refuse to start first try, and sometimes 2-3 cranks are required to even get it to catch. I guess the only good news is whatever is dying looks to be dying faster… I should be able to find it soon, I hope.

 

Jonathan

[jc179]

Worth checking, I'll take the torch and run it around the engine intake and see if there's any chance in its behavior suggesting a leak. I might just yank the intake and replace the gasket, or reseal It at a minimum....

 

Jonathan

[jc179]

Well, as things would have it I was monitoring the fuel pressure when I tried to start today; read 33.6 psi before cranking, and after cranking 0.6 psi. I am not sure what's failing now, could be igniiton switch, the pump or the magic "module" that drives the fuel pump. I need to check both and plan to put a DMM on the output of the "FPDM" where ever that is in the trunk...

 

The haynes manual I have only talks of a relay box in the front passenger fender area, surely my fuel pump control (pwm) isn't there?

 

anyone know where I can find fuel pump brain on a 99/2000 mustang with a returnless setup?

[jc179]

for what it's worth, The FPDM is located in the trunk, drivers side, above the wheel in the fender area. It's held on by 2 10mm nuts, which require a deep socket/wrench to loosen. 

 

... After much messing about, I replaced the fuel injectors in the end, and that resolved the issue. 

 

I also ended up with another issue afterwords, will open a new post.