High revs on start up 99 Mustang cobra

Maintenance/Repairs
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1

Hello everyone. The problem I have is the high revs on start up. In the morning start when engine is 100% cold goes up to 2500rpm and if it warm up to 1900rpm-2100rpm. after starts and it drops immidiatly to 750rpm-800rpm.
Engine is mark VIII 4.6L 89k miles.

what have done before this post:

  1. TPS replaced and adjusted
  2. throttle plate cleaned and adjusted.
  3. IAC replaced
  4. MAS replaced
  5. Coolant temp replaced.
  6. o2 sensors removed and cleaned.
  7. fuel injectors removed, tested and cleaned(interesting fact when reinstalled injectors car first start on cold and rpm went up only 1700rpm.
  8. new air filter.
  9. engine rebuilt and replaced every single gasket except oil pan and transmission + re-timed with new timing kit.
  10. new oil, coolant flashed…
  11. smoked engine for air leaks -all good.
  12. EGR valve removed, tested, cleaned
  13. fuel pressure sensor disconnected and no difference but check came up after.
    no check engine light all around.

I do not understand what else it could be.
Car has aftermarket exhaust but cats still there.
please help on any suggestion.
do not tell me it is normal because exact same car start up 1100rpm.
Thank you.

2

How did you rebuild the engine without taking oil pan off?

3

just the top end seals and all gaskets replaced.

4

Was this high rev on start up happening before the gaskets were replaced?

5

yes from the first day I bought the car.
previous owner installed JLT cold air and I put the stock one back hoping this would solve the problem and after I did all the work you can see above.
my another guess to reprogram ECU…

6

any other sensors could cause this problem?

7

also just checked. car set overnight and there is no fuel pressure whats so ever car starts immediately and even after 4-6 hours off still keep pressure (cannot tell you numbers since I do not have a gage). Testing on fuel rail.
maybe long term leaking injectors?

8

huh? how can a car start with no fuel?

9

when you prime the pump it makes pressure

10

There’s fuel. But it’s not under pressure.

On a fuel injected engine, the system holds what’s called residual fuel pressure.

Residual fuel pressure slowly bleeds down to prevent vapor lock.

But eventually there will be no fuel pressure as the vehicle sits idle.

As long as there’s a column of fuel in the fuel line and fuel rails to the injectors, and the fuel pump runs for the 1-2 seconds when the ignition switch is first turned on, the fuel pressure will be immediately restored.

That’s because you can’t compress a liquid.

Tester

11

I got that, but the OP said it started with zero pressure. But perhaps he means there was zero pressure initially and it came up when it turned the ignition on.

12

I’m not seeing the problem here.