Car idles too fast

[GPModJ]

Been having a problem with the car idling too fast. It idles at around 850 to 950 and runs hot. If I'm in stop 'n go traffic, the temp creeps up very quickly and it's running about 10* hotter than before I started having this problem.

Well, now I'm starting to get a bit p!ssed off. No change so far and here's what I've done:

- Backed the fast idle screw all the way out
- Checked all vacuum hoses twice
- Checked timing & set to 12* without vacuum advance (not connected)
- Replaced fuel bowl & metering block gaskets
- Replaced accelerator pumps
- Replaced power valves
- Cleaned the carb with carb cleaner
- Checked the throttle plates (seem okay--not bent and seem to close all the way)
- Checked the throttle return springs

I was planning to change the front jets to 75s from 76s, as before I started having this problem it was running too rich. So, I went ahead and installed the 75s while I had the carb apart.

I also threw in some new spark plugs. The old ones seem to back up the theory that there's a vacuum leak somewhere, as the plugs are sparkling clean on the center electrode, ground electrode, & part of the insulator. Previously, there were black deposits on just about everything but they're now much cleaner--without my having made any change at all.

Any suggestions?

[Unknown_Authority]

It does sound like a vacuum leak. I didn't see on that big list of stuff you did anything about putting a new carb gasket on. Either that or maybe one of the intake gaskets is leaking. It's usually the easy stuff that throws engines into fits. Good Luck.

[triple66]

I have seen mechanical advance weights stick and give some strange symptoms.

[GPModJ]

Thanks for the replies. I did replace the carb gaskets (one for the carb, another for the nitrous plate) when I had the carb off the car but I haven't checked the intake gaskets. I'm not leaning toward the intake gaskets though because they've apparently been fine all this time.

Do they start leaking out of the blue?

My other thought is whether there's a crack in the manifold somewhere. I haven't seen one anywhere and, again, why would something like that happen?

Triple66, I backed the timing off to 12* but no difference. Stuck or not, 12* is 12* is it not? If the weights were stuck that would mean problems for higher RPMs--no? I'm asking 'cause I don't know.

Just had a thought. I'll try taking the nitrous plate off and see if that makes a difference. Don't think it will, as I haven't seen any cracks in it, but maybe worth a try.

[triple66]

The probelm I have seen is most trouble when it is intermittent. When you rev the engine up, the mechanical advance sticks on (sometimes). Then, you set the timing to whatever you think is best, the idle is good and it all seems correct. Then, next time you rev it up, the advance comes loose, and your timing is now way too low, and your idle is crap. The only way to really nail it down is to set the timing then slowly rev up the engine and let it settle. If the timing varies from try to try the advance weights could be your culprit.

On a separate thought, I have seen aluminum intake manifolds distort enough under the heat to cause vacuum leaks at the ports.

Is your car bright red? And was it at the PPP show at Quantlen college in Surrey?

[GPModJ]

Okay, I've been told by a number of people that I should look at the ports, which is what I'll do tonight. I'll just try tightening and see what happens.

Regarding the PPP show, yes and yes.

[GPModJ]

Found it! :D Someone suggested using a stethoscope to find the leak. Sure enough, I passed the end of the stethoscope over the rear driver's side of the intake and it almost sucked my eardrums outside my head!

So, I guess I'll be taking the manifold off this weekend. Oh well, I've been wanting to clean it up and cut off the water crossover so I can remove the manifold without much fuss in the future.

[THOR]

Good work finding source of vacuum leak Doc !
Is this that beautifully polished manifold of yours ?
What gives? I'm confused why an intake manifold might start leaking.
Gasket messed up? Manifold under or over torqued? Vibration loosen things? Any theories?

[GPModJ]

I'm wondering some of those same thoughts. I was pretty careful to torque everything according to specs when I installed the manifold but a couple of things have been installed / uninstalled over the last couple of months that required removing one or two bolts (not at once though).

I guess I'll get a better idea when I pull the manifold. I'll use a straight-edge to see if the intake runner faces are still straight.

Hmmm... Wonder if gum would seal those leaks in the time being? :wink: Naw, it'd probably just get sucked into the cylinder. :shock:

[Unknown_Authority]

Glad to hear that it wasn't something too serious.

[GPModJ]

Okay, found out what happened to the intake gasket to make it leak. When I took the manifold off, the last four inches or so of the gasket was torn away from the rest.

Both gaskets are identical and can be reversed and used on either side. As a result, they both have holes on both ends for the waterneck. I put both gaskets on without cutting away the unneeded ends at the back of the engine. Moreover, the outlet on the driver's side of the engine has a plug in it so the end of the gasket (i.e. the waterneck hole) surrounds the plug.

When I installed the manifold, I snugged the manifold bolts down by hand and then tightened the bolt that snugs the waterneck to the back of the waterpump, which helps prevent leaks. However, I think when I tightened the waterneck to the waterpump, the gasket must have come with the manifold, putting stress on the back of the gasket.

I'm thinking the gasket finally tore when the engine backfired.

Here's a diagram to help explain.

[Pontiacshack]

Holy crap, now that's giving some detail to the picture. I hear what your saying, I usually tighten the water crossthrough area while the bolts are still loose, and now I know why. What a bummer, but at least you found the problem..... right? Good luck, sounds like your on the road to recovery.