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Old 01-18-2011, 12:13 AM   #1
Plombob
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Default Hard Start Problem '91 Mustang 5.0 H.O.

Iím having trouble finding the cause of a hard starting problem on a 1991 Mustang LX 5.0 H. O.

This is the first Mustang Iíve worked on and Iím not sure what to test next. Any suggestions are appreciated. I donít wrench for a living any more and I hate to think about what Iíve forgotten.

The car sat for about two years before we bought it, but mechanically she was in great shape when we got her.

Some friends and I turned it into a LeMons race car and we ran it hard two days straight. Up until then the car had no starting problems. The problems began almost immediately after the race.

The car wants to start, but doesnít quite make it. If I squirt starting fluid in the air intake, sheíll start. When it does start, it runs rough.

The gas tank had a little bit of what looked like sand. It wasnít much and even if it got past the pickup, the filter should trap it. The filter is new.

Fuel pump is new, and I verified that there is good volume. Tested on both sides of the fuel filter. Havenít determined the fuel pressure. There is spark at the plugs and the plugs were recently replaced, along with the cap, rotor and wires.

Thanks for sharing your experience.
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Old 01-18-2011, 01:03 AM   #2
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

Step 1) Scan for codes.
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Old 01-18-2011, 01:18 AM   #3
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

What have you ruled out so far?

This may seem dumb, but did you refuel after the race? If you did, it might be the gas.
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Old 01-18-2011, 05:45 AM   #4
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

Jumped timing chain? Bump it around to get it to TDC, and see if the rotor is lined up to #1.

When my Bronco jumped the timing chain, it ran great until I stopped and tried to start it again.
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Old 01-18-2011, 05:51 AM   #5
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

Check compression.
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Old 01-18-2011, 05:54 AM   #6
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

Get a fuel pressure gauge on there. Volume means very little. And yeah, check compression too.
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Old 01-18-2011, 07:40 AM   #7
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

Before you spend much time trying to figure this out, do as "Stinger" suggested and do a KOEO test (scanner), and if you make it past that point, a KOER. If you want to check compression, do a cylinder balance test via the Scanner. It will high light what cylinder to check for compression, in the event one of more holes is low. Then go digging for problems.

After that, I checked fuel pressure, Ive removed perfectly working fuel pumps (high mileage), in a "preventative maintenance routine" change out, only to have replaced it with a new "defective" fuel pump. In other words, car runs fine, replace fuel pump, car no longer starts. Or starts, stumbles and runs like crap. Assume nothing. It will save you time and sanity.

Don't feed that motor to much starting fluid, you are asking for problems. Try carb cleaner, not nearly as volatile.

Ive owned and worked on several of the 5.0L Muskrats over the years.
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Old 01-18-2011, 08:07 AM   #8
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

Clogged IAC or faulty MAP sensor have given me similar problems in my prior experience with 5.0's.

Neither one set a code.
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Old 01-18-2011, 08:29 AM   #9
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

hard start hot?
hard start cold?
hard start always?

check your grounds and battery cables
have you tried another battery? perhaps the one you have got bounced around a bit?
coil? are you getting good spark? old coils can degrade and cause problems

bob
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Old 01-19-2011, 12:28 AM   #10
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

Hard start always. I'll check those connections. We have a second battery so I'll swap it out.

Thank you all! This is a big help. I'll get to work on your suggestions and let you know what we find.

Thanks for the tip on the starting fluid. Is that just for the Mustang, or a general rule? I use it to start my 1980 Toy P/U.
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NHBandit says:

I'm just blown away by all the off the wall topics on here sometimes. I need to go find a "Sears sucks" thread to get my head straight...

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The last time that I hit my finger with a cheap dead blow hammer, it hurt just as much as the time that I hit it with the Snap-On dead blow hammer.
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Old 01-19-2011, 08:31 AM   #11
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

Quote:
Originally Posted by Plombob View Post

Thanks for the tip on the starting fluid. Is that just for the Mustang, or a general rule? I use it to start my 1980 Toy P/U.
I start "light" diesels with WD-40 (not all diesels will start on WD40, for example the first gen power-stroke will not), and gas motors with carb cleaner. While Ive never blown a head gasket, or had any other mis-haps with starting fluid, I try to use very sparingly. It will also slow down the starting process if the motor does not fire almost instantly.

Although its about the only thing to get a 2-stroke gas small engine going. Its just a good practice. Ive never had a car that I could not run on carb cleaner. The carb cleaner cant be a decade or 2 old either, it takes a crap, just like gas does. Still cleans fine, but I'm talking about running a car on it.
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Old 01-19-2011, 08:36 AM   #12
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

Spark, Fuel, Timing, Compression.

After that I would look into the IAC and other sensors. But doesn't sound like it to me if it runs like crap at all times.

I've spent a ton of time around Mustangs. Havent had one in a few years though..
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Old 01-19-2011, 09:51 AM   #13
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

Did you confirm the pressure at the fuel rail at startup? Could be the pressure regulator has failed and you're not getting enough fuel.
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Old 01-19-2011, 10:19 AM   #14
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

Check Fuel Pressure.
Need to check it because it may be a bad regulator.
The regulator basically chokes it down AFTER the injectors.
To hold back pressure on it. So it may flow fine but not be able to hold the pressure up.
They are vacuum regulated.

I had an 89 5spd that was supercharged, fun car!

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Old 01-19-2011, 10:20 AM   #15
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

Sorry to repeat Jazz I was typing it up and had to run out for a few... in the mean time you posted.

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Old 01-19-2011, 10:33 AM   #16
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

I would check what rsanter said if it has a hard time cranking. If it starts and runs rough about the only thing left might be the spark plugs wires. You know how to check that, I'm sure. Take it out at night to some dark spot and look to see if you can see the spark running along the wiring.
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Old 01-19-2011, 10:35 AM   #17
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

I would also drain all of that gas out. The sand in it doesn't sound too good, either. Good luck.....................j willis
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Old 03-14-2011, 12:29 AM   #18
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

We followed your suggestions and here's what we found:

We narrowed it down to hard start when cold, and rough idle.

Scanner found only one code: Thermactor system. Didn't find the problem.

Cylinder balance test showed all cylinders are working well. That test was fun to watch!

Fuel pressure test - FAIL! We had a new Airtex pump and it couldn't pump any higher than 25 PSI. Should be 35-45. I remember reading on this board that Airtex wasn't a good brand, so I took Busted Knuckles' suggestion and replaced the pump. We got a Carter rebuild. Problem solved! She starts without touching the gas and runs stronger. Pressure is 35-40 PSI. No leak down either, pressure actually increased to 42 after the engine was off for some time.

We drained the tank and got nearly all of the sand out. There wasn't much.

I expect we'll have better performance at the next race in a couple of weeks. When I drove it at the first race, I thought it should have better performance since it is a police interceptor, but then I thought it is 20 years old.

Here's the thing that puzzles me. The replacement pumps are just a little longer than the original, and we had to fiddle with them to fit the tank. The pump bottoms out on a baffle. It doesn't seem right to me that the clearance is so tight.

Thanks again for everyone's help! I appreciate you helping me get the car ready for race day.

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From Tribbles:

If you're having space issues with this 'too many tools', it means your garage is too small.

NHBandit says:

I'm just blown away by all the off the wall topics on here sometimes. I need to go find a "Sears sucks" thread to get my head straight...

Junkman says:
The last time that I hit my finger with a cheap dead blow hammer, it hurt just as much as the time that I hit it with the Snap-On dead blow hammer.
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Old 03-14-2011, 01:35 AM   #19
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Default Re: Hard Start Problem '91 Mustang 5.0 H.O.

Quote:
Originally Posted by Busted_Knuckles View Post
Before you spend much time trying to figure this out, do as "Stinger" suggested and do a KOEO test (scanner), and if you make it past that point, a KOER. If you want to check compression, do a cylinder balance test via the Scanner. It will high light what cylinder to check for compression, in the event one of more holes is low. Then go digging for problems.

After that, I checked fuel pressure, Ive removed perfectly working fuel pumps (high mileage), in a "preventative maintenance routine" change out, only to have replaced it with a new "defective" fuel pump. In other words, car runs fine, replace fuel pump, car no longer starts. Or starts, stumbles and runs like crap. Assume nothing. It will save you time and sanity.

Don't feed that motor to much starting fluid, you are asking for problems. Try carb cleaner, not nearly as volatile.

Ive owned and worked on several of the 5.0L Muskrats over the years.
KOEO and KOER tests are pretty nice on 90's Fords. I had a 89 Mustang with wierd issues and those tests helped me out.
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