98 Cobra starts but dies immediately - Help Needed

jfisher

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I am officially stuck :)

I've got a 98 Cobra that I recently built an engine over the winter. The full details can be found in my build thread on here, but a quick recap:

98 Cobra
-4.6 4v Teksid block with forged bottom end
-DB Casting C heads with 98 Cobra Cams
-Mach 1 Intake manifold with adapter plate running stock upper lid and stock TB
-COP swap (went back to stock coils and plug wires to eliminate the COP's. Zero difference going back to plug wires)
-Fore fuel rails with Fore FPR
-JLT CAI with stock MAF (and SCT BA 5000 MAF, I've tried both)

That's all I can think of at the moment.

What the car is doing:
-I did break the clock spring and have an airbag light (new one is enroute). Because of this car would initially not start. Just cranked and cranked. Tuner sent me a revised tune file with the PATS turned off and the car will now do what I have typed below
-Will crank and lightly hit but won't start 9 out of 10 times
-Every great once in a while the car will hit, start, rev up to 3k (sounds amazing) but then immediately dies. This makes me think there's no mechanical issue with cam timing, etc (had all brand new OEM timing components installed by a very well-known, pro local engine builder that has done a ton of these)
-I, at one point, was able to get the car running with the MAF unplugged for about a minute, but it eventually died
-Plugs are covered in fuel. They were brand new.
-White smoke coming out of tailpipe after cranking car that overwhelmingly smells of fuel. Car will backfire loudly every now and then like there's a ton of unburnt fuel in the exhaust that ignites
-I have the battery ground cable going down to the driver motor mount stud and then from there to the brake line bracket on the driver side. Verified with a multimeter that I have .9-1.2 ohms when I touch the alternator housing and the battery ground pole

The car ran and drove before I did the built motor over the winter. It was down on power, but it ran and idled. Has a brand new 255lph stock style (funky 98) fuel pump and new fuel filter.

I have to be missing something. What should the vacuum lines look like? What should the grounds look like? Any other suggestions?
 

RAU03MACH

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you Verified with a multimeter that I have .9-1.2 ohms when you touch the alternator housing and the battery ground pole
fresh battery should read 13.00-13.60 to start the car
 

cobrajeff96

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That's a very not good resistance measurement. That short of a run, the reading should be 0.1 or less... as in zero ohms. I would say that's a point of contention and should be corrected soon. It may not be the root cause of all issues, but it's definitely an issue itself. And make sure the alternator housing itself (not the charging stud) is getting good continuity all the way back to (-) battery if you're using the engine as the passthrough. In the absence of a direct lead from alt housing to batt (-), the engine ground needs to be beefy AF and have good continuity to (-) battery for certain.
 
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jfisher

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you Verified with a multimeter that I have .9-1.2 ohms when you touch the alternator housing and the battery ground pole
fresh battery should read 13.00-13.60 to start the car

Car starts sometimes where it is, but just won't stay running
 

RAU03MACH

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could be the wiring for start its like the initial start power wire
like when the fuel pump kicks in it shuts off at first turn
hard to explain but yes it will start for a few seconds and then shuts off
words are on the tip of my Toung.
 

Adfalenski

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if the oil pressure switch is indicating no oil pressure, the computer shuts down the ignition so that the engine oil does not run dry. The oil pressure switch is overridden when cranking so that the engine can start. Once you release the ignition the oil switch comes into play again. It sounds to me like you might have a bad oil switch.
 

ttocs

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I have to agree the ground resistance is too high. Not 10k ohm stupid high, but just high enough to cause other components that use resistance/ground to be off and that includes a lot of things. I recommend using one of the bolts to the starter and make a spot on the frame to ground it to with a strap. The starter pulls more current than anything and is securely mounted to the block as well. I didn't see that your running long tubes so it isn't that hard to get to.
 
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jfisher

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if the oil pressure switch is indicating no oil pressure, the computer shuts down the ignition so that the engine oil does not run dry. The oil pressure switch is overridden when cranking so that the engine can start. Once you release the ignition the oil switch comes into play again. It sounds to me like you might have a bad oil switch.

I will check that out. Didn't know this. It's the same oil sensor that was on the old motor and it worked ok, but can't hurt to check it / the wiring.
 
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jfisher

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I have to agree the ground resistance is too high. Not 10k ohm stupid high, but just high enough to cause other components that use resistance/ground to be off and that includes a lot of things. I recommend using one of the bolts to the starter and make a spot on the frame to ground it to with a strap. The starter pulls more current than anything and is securely mounted to the block as well. I didn't see that your running long tubes so it isn't that hard to get to.

I will definitely take a look at this if not add another ground strap. The starter is pretty tight with the headers. I'm not running long tubes rather Bassani mids, but at the cylinder head / starter area they're very similar in physical size. I do have an extra cable I have saved for making another ground strap if needed. I'm think I could make another the same as the driver side using a motor mount stud and running it over to the brake line bracket on the body.

Appreciate all the suggestions!
 

RAU03MACH

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i at one time tide my MSD box into a wire on the ignition so when the key was turned on power was there but after release the key from start to run it would shut off with no power to that one wire
did you tie any thing into your ignition system
 

ttocs

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the difference between a true ignition wire and an accessory wire is the ignition stays hot while cranking.

For shits and giggles measure the resistance from the battery ground to a clean part of the chassis.
 
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jfisher

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Yep, it was a big vac leak around the intake spacer plate to adapt the B intake lid to the C lower intake. My tuner helped me find it as he added 50% more fuel and the AFR didn't change one bit. Started spraying carb cleaner around the injectors as I thought it was them and that's when the car would jump idle whenever I sprayed it anywhere at all along the intake adapter. Borrowed a pals smoke tester and found multiple leaks around the lid.

Once I got that figured out car ran great. 320whp in April of 2023. Now getting boost.
 

96blak54

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Hey thats great! Glad you figured it out!

Now lets move on to you posting some pics of said car! We all need to see this beast!
 

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