High RPM Idle after Timing Belt Change

After changing the timing belt on a 2.5L V6 SOHC engine (1997 Chrysler
Sebring Convertible) the engine starts but immediately goes to a
redline fast rpm. I don't know where the engine can get the air to run
that fast. I have capped off all the vacuum lines including the power
brake vacuum, the one hooked to the PCV valve and duct taped the EGR
intake on the upper intake manifold (plenum). I capped the throttle
body intake with a aerosol can cap and duct tape; thus no air can go
thru the throttle or thru the air bypass passage. Upper and lower
intake manifold gaskets have been replaced. The intake manifolds
appear to be flat and should seal with new gaskets. The washers around
the fuel injectors are in place. Do I have a magic engine that runs
without air?

If I got the timing belt off 1 or 2 teeth, can air backflow thru the
exhaust?

Can a valve be stuck open?

If I disconnect the MAP sensor and the temperature sensor for the
intake manifold, the engine will not start. I would assume the
computer won't open the fuel injectors without these sensors.

I changed sparkplugs, wires, distributor cap and rotor at the same
time. I screwed up and cross plugged 2 wires. The car actually ran
with the crossed up spark plug wires and would go highway speed but was
weak on power and the check engine light would flash on and off. At
idle I could hear a sound that seemed as if gas was ignited in the
exhaust system. The sound was more off a swoosh than a bang. The high
rpm idle problem started after I corrected the cross spark plug wire
problem.

Any ideas?

could be the IAC....but should run just like it did before changing
timing belt....good luck...


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Scrapper
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Mark3571 wrote:
> After changing the timing belt on a 2.5L V6 SOHC engine (1997 Chrysler
> Sebring Convertible) the engine starts but immediately goes to a
> redline fast rpm. I don't know where the engine can get the air to run
> that fast. I have capped off all the vacuum lines including the power
> brake vacuum, the one hooked to the PCV valve and duct taped the EGR
> intake on the upper intake manifold (plenum). I capped the throttle
> body intake with a aerosol can cap and duct tape; thus no air can go
> thru the throttle or thru the air bypass passage. Upper and lower
> intake manifold gaskets have been replaced. The intake manifolds
> appear to be flat and should seal with new gaskets. The washers around
> the fuel injectors are in place. Do I have a magic engine that runs
> without air?
>
> If I got the timing belt off 1 or 2 teeth, can air backflow thru the
> exhaust?
>
> Can a valve be stuck open?
>
> If I disconnect the MAP sensor and the temperature sensor for the
> intake manifold, the engine will not start. I would assume the
> computer won't open the fuel injectors without these sensors.
>
> I changed sparkplugs, wires, distributor cap and rotor at the same
> time. I screwed up and cross plugged 2 wires. The car actually ran
> with the crossed up spark plug wires and would go highway speed but was
> weak on power and the check engine light would flash on and off. At
> idle I could hear a sound that seemed as if gas was ignited in the
> exhaust system. The sound was more off a swoosh than a bang. The high
> rpm idle problem started after I corrected the cross spark plug wire
> problem.
>
> Any ideas?

I don't have a magic engine. The leak was thru the upper intake
manifold to lower intake manifold gasket. Both manifolds appeared flat
without any scratches or old gasket material. Liberal use of high
temp, sensitive safe RTV cured the problem.

In hind sight I realize I had a small leak before as the idle was too
rough. Once I fixed the leak the car would not idle with out holding
the throttle open. After a few minutes it would idle again. I think
the computer had to relearn the correct idle air position. The air
bypass passage was probably closed due to the previous leak and had to
relearn that it needed to open some in order to idle.

The engine runs good now. If only that transmission ran as well.
After 170,000 miles I guess I shouldn't expect it to be perfectly
smooth.

Mark

Thanks for the reply Scrapper.

At first I thought it could be the IAC, but I blocked off the whole
throttle body intake including the IAC bypass passage with duct tape
and a aerosol can cap. I didn't see how any air could get by that
inlet.

This helped me eventually identify the gasket as the leak.

Mark