rcchevrolet

At idle, the RPM needle is oscillating up and down about 20-60 RPM (very noticeable). With the engine running, the foot on the brake pedal has almost no oscillation.
I have lost about one MPG in fuel efficiency lately from 16.4 down to 15.4 MPG suspect maybe the needle fluctuation has something to do with it.
Any and all comments would be greatly appreciated.

hanky

RPM is usually reported by crank sensor. Maybe loose connection in wiring circuit , not enough to prevent starting , but enough for tach to pick up on the problem.

rcchevrolet

Thank you for your very valuable information. Will be checking the CKP sensor, visually as you suggested then will google to find out how to check the harness side connector and how to test the sensor also.
Before posting here I cleaned the IAT/MAF and MAP sensors, not sure but, needle movement seems to have increased notably.
Will post my findings.

rcchevrolet

Thank you for your very valuable suggestion. Will be visually checking the CKP sensor as you suggested if nothing is found then will google how to check the sensor and the harness side for power. Before posting here I cleaned the IAT/MAF, CKP sensors, and throttle body properly, and am not sure but believe the idle RPMs have gone down from about 900 to about 650 RPM, which I take as a good thing.

ruley73

It's not a bad idea to make sure the crank position sensor is clean and properly seated, but it's probably fine unless it's been removed before.

What's the status of the spark plugs and wires? Have they ever been replaced? If not, they're overdue as 100,000 miles is the recommended interval. The OE plugs are iridium so you can go a little longer, but eventually gap erosion will take it's toll in the form of decreased performance/fuel economy and misfires if they're left in there too long. The wider gap also puts more strain on the coils, but I've never seen one fail on these engines. It is also strongly recommended to replace the wires when replacing the spark plugs on this vehicle/engine.

You could also check the vacuum lines (namely, the brake booster fitting at the top of the intake plenum and both PCV hoses) and the EGR valve pipe. I had to replace the EGR pipe a few years ago on ours because a crack developed in one of the seams. It caused a very audible exhaust leak, but didn't throw any codes. You could remove the EGR valve and inspect the port in the intake plenum for carbon buildup, but it's probably fine too if there are no EGR-related DTCs.

I wouldn't worry about the fluctuation too much if you're not seeing a check engine light and everything mentioned here checks out fine. The fuel economy difference is negligible IMO - especially if you're just going off the MPG reading you see in the gauge cluster. I've found that it's usually off by +/- 1 MPG when compared to actual math at fill ups.

rcchevrolet

This morning's drive shows the following changes: Idle RPM is down to 500, and RPM needle oscillation is now much reduced. Perhaps by completing two driving cycles after the cleaning of sensors has registered with the computers. Just learned from my wife that the oscillating needle has been like that for a very long time. Replaced spark plugs and cables last August 2022.
Have owned this SUV for seven (7) years and it has been very reliable except that it uses a lot of engine oil, about 1 qt every 800-900 miles, and had a lot of undercarriage rust which I have kept under control. Being an older, retired person the car sees very low use, with about 2500 per year. The EGR is the only component that has failed. The engine has plenty of power plus we tow a small, 2800-3000 lbs, Pop Up camper (with an extra transmission fluid cooler I installed) without any issues.
May be able to check the CKP sensor this weekend and will report. Thanks to both of you for the great responses.

ruley73

That's not good. These old pushrod engines rarely consume oil like that. I would try cleaning the PCV valve just before the next oil change if you haven't already done so. You can't replace the PCV valve without replacing the entire valve cover so just remove the hose from the left/front valve cover and shoot a generous amount of brake cleaner through it. Then drive it for a bit before changing the oil.

You may have an internal leak somewhere (probably lower intake gasket) if you're not seeing an external leak. FWIW ours burns hardly any oil. I usually have to add a half quart between oil changes (usually ~5500 mile intervals), but it's had a minor external oil leak from an unknown location for the last few years. We've had it since 2008 and it currently has just over 212k miles. It's had no major issues, but the lower intake gasket just started leaking coolant externally around the rear rail seal. I plan on fixing that soon before it gets too cold. Rust from midwest winters is taking it's toll on the undercarriage. We'll probably get another year or two out of it, but anything beyond that would be surprising. It's been paid off for years so a $500 repair here or there doesn't bother me. That's still cheaper than a couple payments on a new vehicle.

rcchevrolet

ruley 73:
QUOTE: "It's had no major issues, but the lower intake gasket just started leaking coolant externally around the rear rail seal. I plan on fixing that soon before it gets too cold."
Are you referring to rear fuel rail seals? If you do your repair could you share the experience? It looks very difficult to me.

Have removed and inspected the CKP sensor and, am happy to report no rusted connection or bolt looseness.
One thing I noticed because of your above-quoted comment is that I do have some fresh engine oil leak at the back of the engine, upper part, visible at the lower edge of the exhaust manifold heat shield but not further down and not leaking onto the ground. Because of the heat shield, I can't see where is coming from exactly.
Thank you both for the great help

ruley73

No, I'm referring to the two lip seals that follow the front and rear edges of the block between the intake to cylinder head plate gaskets. They're the orange pieces in the gasket kit linked here.

rcchevrolet

Thank you very much.