Problems diagnosing cold start issue-95 mustang gt

[Rizwan]

Hi Guys,

I am new to this forum. So here’s what is going on with this mustang. It gives me a lot of trouble starting up when the engine is cold. Every time I start the engine, it shuts off…so I keep trying to start the engine. After about 8 or 10 starts, the engine finally stays on but then the idle goes up and down and up and down. While it does this, somethings the engine stalls while the idle goes up and down (I think the correct term is surging). After the engine reaching normal operating temperature, the idle becomes more stable but it still goes up and down slightly here and there.

So it looks like I have to problems:
1. Engine dies immediate on a cold start and
2. Idle surging

Here are all the thing that I have checked for in the past couple of days which we can eliminate:
1. coolant temp sensor
2. throttle body is clean
3. Mass air flow sensor is new
4. intake air temp sensor
5. It is a newly rebuilt engine so I don’t expect dirty intake valves
6. fuel pump and fuel filter is brand new.
7. egr valve and egr position sensor

I think this leaves us with vacuum leaks, IAC valve, and fuel pressure regulator.

I will admit one thing. Sometime ago, when I got tired of the engine dying on me, I messed with the base screw of the throttle body. I turned the screw clockwise which made the throttle plate open more which temporarily helped the engine stay on when I started it instead of dying immediately. I was reading somewhere else that this messes up the iac setting and that it has to be reset. Is this true?

With regards to vacuum leaks, I have tried the telescope method and could not find any leak. But I have not yet tried the spray bottle (soapy water) method or using carburetor cleaner spray. I will try this tomorrow.

I will have to try testing the fuel pressure regulator tomorrow also. I was reading on how to test the fuel pressure regulator. The article said to unplug the vacuum line and watch if the fuel leaks out. Where would the fuel leak out from: the vacuum hose or the regulator? Also do I do this with the car on or off?

Are there any good detailed tips on how I can check if the IAC valve is good? BTW I have already cleaned it. Also I don’t have a scanner tool that gives live data.

Thanks for all the help in advance!

Topic

[Dylan Gilbertson]

I would guess your IAC valve, but if theres a vacuum leak near the intake it’ll do the same. My dad taught me to take some floss or some thread and hold a loose end around all the hoses, if theres a leak it should move. Or take a hose, put one end by your ear and the other end to listen for the hissing.

[Rizwan]

Thanks. But do you know of a good method to check if the IAC is bad without using a scanner tool?

[WOTStang]

Here is a ‘shade tree’ method of testing the IAC valve that a few mustang guys have used in the past.

– remove IAC and gasket from the side of the throttle body.

– Take thick tape (or a helpers hand) and block off the two ports to the side of the throttle body (leaving the IAC plugged in, as you can now watch if it moves, and also to not set a check engine light).

– start the engine with the ports blocked off, and see if there is a change in it staying running.

I know a guy who cleaned his throttle body once with brake clean and it got into the IAC valve and it deteriorated all of the rubber seals inside and it either didnt want to run at all, or idle at 1500-2000rpm. I did this same test and it ran well after that (it probably wont stay running well, as it is still expecting that feedback from the IAC, but it should stay running while cold.)

As far as a more technical method of testing.. i can get you the resistance values and voltage tests to see if it is indeed operating properly.. but i have used the above method before to diagnose a failed valve.

As far as the failed fuel pressure regulator.. when you pull the vacuum line off, you will definitely smell gas on the hose, and if its bad enough, you will see it for sure. however, symptoms of a failed pressure regulator usually lead to extreme rich conditions (fuel out of the tail pipe, blackened fouled plugs, check engine lights for rich conditions (although OBD1 is not as particular about rich/lean as OBD2). But then again, a hard restart when cold (fuel pressure draining off) could also be a symptom of a bad regulator (or check valve in the fuel pump).

[Rizwan]

Thanks for your reply. I have not had a chance to try this yet but i will soon.

[Rizwan]

Thanks for all the help. But I found out that the problem was the after market MAF Sensor. So i went to the junkyard and got a genuine one and now it starts up when cold.

But it is still surging when I start the engine. Any ideas? I think it is a vacuum leak but i can find it.

[EricTheCarGuy]

Vacuum leak or IAC valve problem are at the top of my list. Especially since the engine was just rebuilt. It could be that the vacuum lines aren’t connected correctly, or there is a leak somewhere. Either way, it sounds like a vacuum leak to me. BTW, the IAC would also technically be a vacuum leak if that is the issue. My method of testing is about as back yard as you can get. While the engine is acting up, smack it with a screwdriver handle. If the idle changes when you do this, then I would consider replacing it. More info here.

http://www.ericthecarguy.com/faq/solving-automotive-performance-issues

Keep us posted.

[Author]

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