Hesitation after 1/4 temp guage AND constant speed

[Gold_BM]

Hi.

My 2000 SL1 hesitates after it gets up to temp (a hair past the 1/4 mark). Furthermore, this ONLY occurs at constant speed. I solve the hesitation by accelerating.

As the original owner, I have 226K miles.

I just followed Richpins P0410 Secondary Air troubleshooting, and successfully cleaned out all the Carbon in the pipe leading to the intake. The Air Pump runs fine. The outlet of the Diverter valve shows flow, but seems much weaker than what the Pump is pushing before it goes through the Diverter.

I have never replaced the O2 sensors.

Thank you.

Ed

[OldNuc]

Did you ream out the little air holes in the head?

As you had the exhaust off it is a good idea to check this for leaks as a leak can cause a hesitation when the engine is warm.

Exhaust leak check.
--Locate a low pressure high volume air source, reversible vacuum cleaner, large fan, leaf blower or what ever you can find.
--Loosely couple to cold exhaust at the tail pipe, or direct fan at tail pipe.
--Spray every inch of the exhaust between the head and CAT outlet flange with a mixture of 2 or 3 drops of dish soap and water in a spray bottle.
--Pay close attention to the lower flange, lower support clamp, and flex connector.
--The flex connector is under the protective braid so it requires quite a bit of solution to show any leaks.

Check everything listed.

Rear O2 sensor has no control function so leave it alone and the front will let you know when it fails.

[fdryer]

Was the original coolant sensor ever replaced? Original round nosed resin sensors failed by cracking, sending incorrect temperature signals to the pcm. The pcm senses freezing temps and runs the engine rich all the time. Replacing it with a flat nosed brass sensor corrects this problem. How's heater output?

[Saturn Night]

Originally Posted by fdryer

Was the original coolant sensor ever replaced? Original round nosed resin sensors failed by cracking, sending incorrect temperature signals to the pcm. The pcm senses freezing temps and runs the engine rich all the time. Replacing it with a flat nosed brass sensor corrects this problem. How's heater output?

+1. Also pull a spark plug and check its condition. The ¼-mark coolant gauge is a tell-tale sign of a faulty ECTS or faulty thermostat, and the plugs should be fouled and black-sooty in appearance.

You are getting a hesitation, so take Fdryer's advice and pull the ECTS sensor, which is located under the EGR valve, and requires a 13mm deep-well socket to pull out. You will lose a small amount of coolant.

Check the wiring for corrosion.

[Signmaster]

I'd check all the basic tune stuff. Wires, plugs, ECTS, electrical connections. Some hesitations and misfires will only happen once the mixture is leaned towards normal, and warm up mixture will mask them.

Often on my single cammer, any indication of plug wear, etc only happens once up at or near operating temp and usually under light loads. Throttle tip in from a 40-45 MPH cruise or similar at higher speeds. Throttle application will usually mask the misfire.

[Gold_BM]

Thank you all. I've got my work cut out and will address in order of "Homeowner importance". I thank God to this day I bought this Saturn 18 years ago.

Ed