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BOSS 5.4L 260 Going into limp ode after the car reaches temp…

BOSS 5.4L 260 Going...

BOSS 5.4L 260 Going into limp ode after the car reaches temp as the car is coming up to temp its fine the fans come on the temp goes down the fan goes off after this the revs start increasing and drops and goes into limp mode any help would be great

Mechanic's Assistant: Will the headlights turn on? Is the battery fully charged?

Yes new battery

Mechanic's Assistant: What is the make/model/year of your car?

The motor is out of a 2003 XR8 the car is replica GT40

Mechanic's Assistant: Are you fixing your vehicle yourself? What have you tried so far?

New temp sensor second hand throttle body reset the PCM left the battery unplugged

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

The temp sensor was originally replaced because the fan did not come on and I am just hoping I have not created this problem when I replaced the temp sensor but the fitting was correct and it was advertised as suited for this motor.

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Answered in 2 minutes by:
1/15/2018
Gary
Gary, Australia Car Mechanic
Category: Australia Car
Satisfied Customers: 1,499
Experience: Senior Technician
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Hello! My name is Gary.
Welcome to JustAnswer.
I'm reviewing your question now, and will post back with a reply shortly.

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Hello there,

Did you do the conversion wiring?

Your GT40 is a manual? Is the donor PCM from an an Auto or Manual trans vehicle?

Gary

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Customer reply replied 1 month ago
No the loom was supplied by the company supplying the kit roaring forties in Melbourne, I did not build the car I bought it already built
Customer reply replied 1 month ago
Sorry i it is manual and it has a modified tune by SCT

Ah ok, there is something amiss with the tune I'd say.

I would contact the tuner first.

Have you checked some basics like chassis grounds, and 12v feeds for any poor connections? You might find a connection physically "hot", which is a voltage drop.

I bet this little thing is a rocket with a quad cam in it.

Gary

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Customer reply replied 1 month ago
I checked voltages to the PCM for voltages checked all connections, check all earths including continuity to make sure good earth to engine etc all seems ok. Have not noticed any connections getting warm yet, but something else to check.
Yes when it was running it sure does get along.
Customer reply replied 1 month ago
The SCT flash tuner came with the car so would of guessed this keeps the modified tune and can be refreshed I did this when I noticed the fans did not come on when the car came to temp. I hope this is the correct tune and I have developed this fault by refreshing the PCM
Thanks
Stuart

Hi Stuart,

If the fault happens everytime it gets to operating temp, it could be as described above, which I have found a few different cars with "hot" connections, one was so hot it burnt my finger, not funny at the time.

A scan for codes would be the next step.

It may be a throttle body issue, or circuit wiring for throttle body.

Mid engine, low centre of g, must be fun.

Gary

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Customer reply replied 1 month ago
The SCT tuner has a feature to detect DCT but nothing comes up, but I will go through connections as you have described, this is the 3rd throttle bottle, I had the original which swapped with an aftermarket SS Induction unit, I thought that may have caused the issue and tried another throttle of a known good working one from another car. Rest PCM still goes into limp mode. Just for my own curiosity I am going to replace the ECT (OR CHT not sure which one this motor has) simply because the gauge reads 80 odd degrees but if it is a CHT this measures the physical heat of the head itself as far as I can tell. If this sensor reporting the reading back to the PCM it could be causing an issue.Have not driven the car much as I had to replace the suspension bushes as they perished cannot wait to get running again maybe take on a track day at Goulburn circuit
Thanks
Stuart

That could be a parameter that the tuner decides on.

CHT is generally higher than ECT.

A thread on the subject:

https://www.hptuners.com/forum/showthread.php?62438-ECT-vs-CHT-coolant-temperature-readings

Gary

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Customer reply replied 1 month ago
OK thanks thats what I thought,

Ok Stuart.

I'll leave it with you for now.

When dealing with custom setups, there is always some freak one off abnormality as the cause of ones woes.

I had a mapping issue a few years back, that I couldn't get around, and even a dyno operator couldn't see the error in the data. Then I had a light bulb moment, went into data, changed the scaling for MAP, different car straight up.

Back on the dyno, got my lost 100hp back.

Yours being custom, I hope the loom is correct for current flow etc.

Gary

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Customer reply replied 1 month ago
Thanks Gary

Hi Stuart,

This is a fairly complex issue, sorry I can't offer any more at the moment.

At best, ***** ***** say the tuner needs to look at what is going on here.

Hopefully the fault is reproducible for him.

Kind regards

Gary

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Customer reply replied 1 month ago
Hi Gary
Thanks for your assistance I did not think it was going to be a straight forward problem, but I do have an update I just re-flashed the PCM using the modified tune (I think I used the strategy tune last time). The car idled perfectly fans came on like before, stayed on until the temp dropped a few degrees and then came back on as the temp increased slightly all normal.However after this the rpm's increased from idle to around 1200 rpm and it seemed to hunt a bit. Had the car running for about 1/2 an hour and I just switched it off but the good news it did not go into limp mode during this test. It seems like the PCM is still learning I might disconnect the battery again to reset and go through the whole process again.Cheers
Stuart

Hi Stuart,

That's good to hear.

If the car is now driveable, I'd take it for a short drive so some fuel trim learning can occur.

Do you have a working engine light as part of your instrument cluster?

Gary

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Customer reply replied 1 month ago
No I am not sure why they did not include this I will see if can work this out and rig up a light its a good idea.Thanks Stuart

Yes I would do that.

I've done it a few times on mine and mates conversions.

It's a good talking point when you key on, and your friends see an "engine light".

They assume you have quad webers.

Gary

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Hello again Stuart,

Do you need any further help?

I hope my advice has been of assist to you.

Kind regards.

Gary

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Customer reply replied 1 month ago
Thanks Gary that's all thanks I have a call to another GT40 owner who is a mechanic as well I see if he can assist.Cheers
Stuart

Ok Stuart.

I wish you well.

Gary

Gary
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