My 90 CC's 307 has decided to act weird and start surging in idle. It revs high as normal with the choke on until you kick it off but then it doesn't drop down all the way like it used to. The idle is faster than normal and the choke is open. It doesn't surge then though. Once it's been driven about 30 minutes and shut off it will surge when restarted. Sometimes a throttle bump stops it other times it will keep doing it until in gear. I have a tiny coolant drip from the rear of the intake manifold, about 2 drops every 4 days if not driven. It's not new, but could that cause enough vacuum loss to have this affect?

 

The coolant leak shouldn't have much to do with any vacuum issues unless it's somehow leaking into the intake ports, but very very unlikely. But, that leak won't fix itself. You're going to have to replace the intake manifold gasket and check the heads/intake area for galvanic corrosion pitting. Nothing new with aluminum intaked 307s, especially the peanut port ones. Intake gasket set is p/n Fel Pro MS96039. Or equivalent. Find them for around 15-20 bucks. (GM p/n 22535437)

Did you check the carb hold down bolts for tightness? Any check engine lights? Vacuum hoses coming loose or cracked?

 

No lights, I did tighten the front two carb bolts but no difference. No obvious hose leaks but I need to go through all of them. It is completely stock original factory parts. 123k miles.

 

The best thing to do is measure the vacuum w/ a vacuum gauge & evaluate the results. A properly tuned stock engine should achieve a STEADY vacuum of ~17"Hg - 21"Hg.

 

Where should I connect the gauge? Will any vacuum point work?

 

Connect the gauge to any intake manifold vacuum port. You can insert a plastic "T" in-line to a existing intake manifold vacuum port or remove a existing vacuum line and connect directly to an intake manifold vacuum port (cap the line you removed).

 

Try tightening the intake manifold bolts for the coolant leak. For the surging you need to follow the tune up procedure for timing and idle mixture.

 

Check for stored CCC trouble codes. See if a local garage will smoke test it for vacuum leaks. That model/engine/year probably has the most emission related amount of vacuum hose of any car ever made; but, when everything is correct they run well.

As far as carburetor work/adjustment goes don't allow anyone without experience on CCC carbs touch it. A factory manual is truly a must if performing carb adjustments/repairs.

 

That's a fact. Insist on a professional card-carrying CCC mechanic.

 

My Dad was a GM Master Tech at Royal Oldsmobile in Metarie Louisiana in the 60s, 70s ,and 80s but he retired in 1991 and has since passed on. He was the only CCC tech I still knew, so I can't imagine finding one now would be an easy task.

 

See what you can find by checking for CCC trouble codes, using a vacuum gauge, confirm the ignition timing is good and do a smoke test for vacuum leaks before doing anything to the carb. The carb usually isn't the problem. If you venture into the carb have a factory manual on hand and a dwell meter or a duty cycle meter and follow every step in order.

Check the purge valve for raw fuel in it, there shouldn't be any, these had a high failure rate.

 

The EGR valve can certainly cause a surging condition as well. Check to see if it is sticky in operation or not seating completely.

Rick

 

Tightening the bolts for the intake MIGHT buy you some time for that leak, but I've seen it more often than I'd like to report. For some reason it was only on the 7A head peanut port 307s. Galvanic corrosion is wreaking havoc on that area of the intake. It will need mechanical intervention to make it right.

A lot of good places to start were mentioned here. I suggest you compartmentalize all the vacuum sources and where they go and try to isolate one at a time.

You may also have "hidden" vacuum leaks. Canister control valve (CCV) plastic vacuum operated diaphragm could leak, for example, causing vacuum problems and you will never see it. Check every vacuum operated diaphragm for proper vacuum hold. I've had CCV TVS valves literally pop their top (just glue holding down a spring loaded plastic cap) and cause a vacuum leak that's not instantly obvious.

If you have a positive pressure EGR, you'll ALWAYS have a vacuum leak if you try to do a vacuum check on it without the engine running at a good RPM. It's just how it reacts. It needs exhaust pressure to operate. This is where a CSM comes in handy for troubleshooting all the emissions stuff.

Another possibility, is the ILC system. Although it's NOT supposed to hunt, or surge, because there's a delay valve in there, if something isn't working right, it may cause issues. I'm not very fluent in ILC detailed troubleshooting because 98% of my CCC carbed cars are/were VIN 9 307 cars and only had ISS to worry about and no ILC. I owned one 87 Salon with the ILC a long time ago, but it was low miles and ran excellent, so no need to dig into it.

 

There's so many vacuum parts, it's crazy. I only drive it once a week and we're putting a fence up this weekend. So I'll have to get to it next weekend. I do have the factory manual and always consult it before doing anything and if possible use NOS parts. Because I plan to do the manifold gasket I have found every emissions part over the last year except an ILC. Just in case they break removing the vacuum hoses. I'll have to make a list from everything you guys suggested and go through it. Thank you.

 

I would expect to find significant pitting on the intake manifold mating surface particularly at the coolant passages. I'm wondering what members have to done to repair the pitting? Is something like JB weld filed smoothly a viable option?

 

Exactly how I've seen it repaired before. Clean up the intake area, take some JB weld, a putty spreader, fill in the pits, let cure, lightly sand smooth, reassemble. Never seen another leak from a repair like that.

 

I recommend using Mr Gasket composition intake gaskets instead of the turkey tray thing if there is corrosion at the water ports. It's worked for me and still is.

 

Yesterday it decided to be hard to start, took quite a while cranking for it to start cold, like almost two minutes off and on. Choke is working correctly. I did notice as it warms up the plate does not go full 90 degrees vertical like it's weak, it's close but not straight up. Last weekend used it to do two utility trailers worth of mulch (2 yds) but no problems. No coolant leaked out this week either. No milkshake either, thank God.
How often do you change the fuel filter? The manual doesn't say. The 70 98 says 12 months but nothing for the CC.