High Idle Problem Identified (98% certainty)!

[Bushwack]

For some time I've been asking for advice how to resolve a high idle problem with my '90 Reatta (to be referred as Reatta #1). If you recall, I've changed various sensors, IAC, thermostat, cleaned the injectors, PCV valve, kitchen sink - the list goes on.

Last Sunday & Monday, I took out my seldom driven Reatta for a spin (let's call this Reatta #2 - it's in my signature). Car ran great from a cold start through reaching normal temperature. Since it's been 5 months (310 miles), I took the car in for an LOF. Four hours later when I picked up the car and drove it home, I noticed upon braking it felt like the car was also accelerating. I took a temperature reading (140*), then accelerated down the block to 25MPH, put it into neutral and the tach went up to 1,500RPM, only to drop down to 900 upon a complete stop (same behavior as Reatta #1). I did this many times with the same results. It wasn't until the car reached 186* did the idle drop to where it should be (740-760RPM).

So...Valvoline 5W30 is NOT the oil of choice for my Reattas (anyone using Valvoline?). It seems with 98% certainty that the problem is the oil (or oil filter?). He did find a couple similar circumstances on-line regarding a 1989 Park Avenue & a 1990 Riviera - both problems originating after an oil change (didn't say what brand oil or the weight). Mechanic is thinking of going to a 10W30 but he will first research further and call the GM tech line for further information.

[Guest Richard D]

I use Pennzoil dino, 10-30 change it every six months (about 2,000 miles) almost at 147,000 miles with no problems.

[padgett]

I have always used 10w-30 in Reattae but this is a warm climate so permitted according to the owner's manual. No idea why 5w-30 would cause a high idle though.

Edited November 15, 2012 by padgett (see edit history)

[Bushwack]

Very puzzled why 5W-30 would be the cause considering it has been used previously (but unsure if it was Valvoline or another brand). I'm hoping the GM tech has something thoughtful to add. But no matter what he says, at the very least I need to use a different weight and brand (if just to confirm the oil is the problem).

[Rawja]

Just remember correlation doesn't necessarily imply causation. What you're reporting doesn't seem to make any sense.

And as a side note, when I lived in NY, I had a relationship with my local Valvoline quicky-lube, always used them, always used their 5W30, never had any issues.

[Ronnie]

Just remember correlation doesn't necessarily imply causation. What you're reporting doesn't seem to make any sense.

I agree. The odds that an abnormal fast idle is being caused by the brand or weight of oil is very low in my opinion. I would concentrate on what might have been done during the maintenance procedure that would cause the problem. Was the PCV valve removed for inspection or replacement is the first question that comes to mind for me...

[NCReatta]

Yeah, in the words of Click and Clack: Booooooogus.

Oil is simply a lubricant. There doesn't seem to be any way that it could be connected to a fast idle.

However, Ronnie may have a point. There might be something else they inspect when they do the oil change that could be causing it.

[Bushwack]

I don't disagree with anything that's been said. I still can't imagine the oil being the problem. But the fact is the car ran fine until the oil change. Like a curious child, yesterday I went step-by-step with the technician on the process of changing the oil. It wasn't anything out of the ordinary (don't know if the PCV valve was inspected but with the other Reatta, that has been replaced without solving the high idle on that car).

Hope to know more by the end of day.

[Bushwack]

The results are in and...

There are three documented calls (since 1994) to the GM Tech line of a high idle after an oil change with the 3800 engine. But so few calls doesn't warrant investigating further.

I had 10W replace the 5W-30 and the car does start up and run smoother for the crucial first few minutes. But while I agree that lubrication isn't the reason for the idle being either high or normal, there's something in the LOF process done by the mechanic that caused the problem. I do feel some progress has been made as, after replacing/cleaning various items under the hood, I feel I have a starting point when (how) the problem occurred. I did look back on invoices for the other Reatta and sure enough, I first brought up this problem last June to my mechanic 12 miles after a LOF (done by the same technician).

So...this guy isn't working on these cars anymore but I still have a high idle to resolve (for Reatta #1). Ongoing thanks for everyone's help.

[Ronnie]

I don't quite understand the results as described. To be more to the point: Did putting 10 weight oil in the car correct the problem with the high idle while the engine is cold?

[Bushwack]

Yes. But I don't think the 10W is why the idle is back to normal.

[Guest 89 Maui]

i tend to agree that there should be no real operating difference between 5W and 10W except in very cold climate. In reviewing the changing of parts which did not inprove the problem I am looking to what did make the change in improvement and it comes down to the oil and .... the oil filter. If the oil filter has a defect and is affected by the cold oil there should be a drop in oil pressure but increasing the temperature of the oil would have an increase in oil pressure.

The question becomes what in an oil filter could cause this, 1. maybe a double filter, 2. excessive adhesive, 3. safety bypass problem, 4. or ......

What was the manu of the oil filter, too late to take the old filter and have it examined unless you kept it.

[padgett]

Doubt it, there are two bypasses involved. More likely in the course of the LOF "something" else happened such as a knocked off vaccuum hose, that will foull up the idle.

[ptt]

Not saying your mechanic is this way but Ive encountered a few grease monkeys over the last 50 years that liked to "make their own work" above and beyond what they were asked to repair! Everything from recirc-fuel lines left off the charchol canisters to sparkplug wires left dangling causing everything from fuel smells to engine misses....and a return trip to the mechanic. Its the very reason I started doing my own repairs.:eek:

[Guest astro355]

I had this same problem with my Reatta when I bought it. A through cleaning of the throttle body, MAF and IAC resolved the issue. The was carbon build-up on the back of the throttle blade and if I had to guess, made the blade stick open more than it should.