My idle is "throbbing"

[MMT51]

I tested the MAF, using a sewing needle to backprobe the connector. I followed JimBee's instructions from the thread cited above. Where he advised that at idle the voltage at Pin 4 should be about 1.0V (.8V to 1.2V), I got 1.4V. At 3500rpm it only rose to 2.0V (JimBee advised about 4+V). The MAF has 3 years and 15,000 miles on it.

I put in a new Bosch MAF and disconnected the battery to reset the computer. The result is smoother starting, smoother acceleration, a little more power, a little less vibration, but the same RPM fluctuation at idle (c.830-870). The car is nicer to drive, but not fabulous.

I'm thinking about:
1) Seafoam
2) Change plugs to Volvo brand
3) Send out injectors for service

Does this sound sensible? Having eliminated vacuum leaks (I hope) and changed out the MAF, where else to look?

[MMT51]

Still struggling with this wandering idle (830-870 RPM). Doesn't want to go above about 2800 RPM without kicking down the gas. Car has poor acceleration and not much power.

I replaced 4 of the 5 ignition coils due to cracked housings and put in new Volvo spark plugs.
Cleaned the intake with Mopar Combustion Chamber Cleaner (foam) in the manifold, and Seafoam in the crankcase. MAF is new, vacuum leaks fixed, front O2 sensor new in the last 18 months.

A vacuum gauge on the manifold reads 19 inches of mercury with a quick 1 inch drop every couple of seconds.

It still starts easily, never stalls or hesitates, and no codes. I'm running out of ideas. Tomorrow I'll test the fuel pressure.

I'm grateful for any suggestions.

[cn90]

- ICV Valve: is it clean?
- Check every single vacuum hose with a flash light.
Be slow and methodical to go over all vacuum hoses.

[mercuric]

Is it just at idle, or if you rev it up a little, say 1500rpm, do you still get the "throb" and observed vacuum drop?

I'd be a lot more suspicious of the IAC valve if the problem mainly presents with the car at idle. I'm not sure how the newer cars operate, but on RWD bricks if you just unplug the IAC, you end up with a high idle... but if that's suddenly a smooth idle, it's a pretty good clue.

IAC valve will certainly cause vacuum blips if sticky, but that often points to individual cylinder firings not going as they should due to any number of reasons, especially when consistent through the RPM range and when the blip pattern follows engine speed. injectors, ignition system components, and valvetrain issues top the list.

You mentioned you replaced 4 of 5 coils -- Does swapping one of the previous coils in place of the one that hasn't been replaced change anything? Just a troubleshooting thought...

You might also try cutting fuel to each cylinder individually and compare results -- if cutting fuel to one of the cylinders results in weird behavior compared to the other cylinders, you can focus on that cylinder for troubleshooting. Not sure how the newer FI systems react to this, but the older 5-bangers will run on 4, not well, but enough for analysis purposes.

I'm also a believer in off-car injector service -- ultrasonic cleaning, replace filters/seals/pintle caps, run flowbench numbers to confirm equal flow. I do this as preventative maint / efficiency optimization once when I get a new to me brick -- can net smoother engine operation and increased fuel efficiency if the spray pattern was lacking. I also like knowing the injectors are flowing equal fuel to each cylinder, which is quite important on a turbocharged motor. Probably not a fix to your issue unless the cause is a sticky injector, though, and cleaning wont fix intermittent actuation problems if it's electrically-related.

[JRL]

I had a 99 like that a year or so ago, minor throb and very minor RPM fluctuating at idle only
It was my winter driver.
Had about 110K miles

I replaced just about everything!
All motor mounts
MAF sensor
02 sensors and harnesses, etc
It got better and it really was very minor. Most people would never know or say anything (the buyer never did and is happy with the XC), but it drove me nuts as I KNOW how these cars usually are.
Bottom line it was never fixed 100%

[bw77]

A vacuum gauge on the manifold reads 19 inches of mercury with a quick 1 inch drop every couple of seconds.

Quick drop of vacuum suggests a valve problem.
Have you done leakdown and compression tests?

Check out this page about vacuum readings:

http://www.international-auto.com/fiat- ... gauges.cfm

[JRL]

We did literally everything and more, nothing!
Everything was nominal or better, yet it persisted

[abscate]

Couldn't you perhaps firm that valve diagnosis by bring the idle revs up by 2x and seeing if the vacuum dip frequency doubles?

[MMT51]

Thanks to all for suggestions. I did some more testing, now I need more advice. 2000 S70 GLT SE with 139,000 miles.

- ICV Valve: is it clean?
- Check every single vacuum hose with a flash light.
Be slow and methodical to go over all vacuum hoses.

This engine has no ICV (due to Electronic Throttle Module?). I had been searching for this for quite awhile before the parts guy at the dealer looked it up for me. I'm going to replace the 3 hoses from the Turbo Control Valve, but my smoke testing hasn't shown any more leaks.

You mentioned you replaced 4 of 5 coils -- Does swapping one of the previous coils in place of the one that hasn't been replaced change anything?[/quote

The 5th coil was new 8000 miles and 2 years ago and was the only one without cracks.

Manifold vacuum

Idling - 830-870 rpm: 19 inches of mercury with quick drops to 18 inches every few seconds
1500 rpm: 21 inches steady
2000-3000 rpm: 21-22 inches vibrating
WOT – drops to almost zero and then back to about 21

Injector harnesses - tested with noid light: steady signal at all injectors

Fuel Pressure at fuel rail: 50 psi with ignition on / 56 psi with engine idling (Haynes manual says 44 psi).
Is too much pressure a problem?

Compression Test
1 190
2 190
3 180 (190 if I keep cranking a few more. Leaks back down to 180 in a few minutes)
4 195
5 195

I tested #3 with oil in cylinder and got 200, which slowly dropped to 190.

Thanks in advance for any help interpreting these results.

Peter

[JRL]

On a 135K mile engine with one cylinder not quite as good as the others, that could be just enough and there's not a heck of a lot you can do about it.
Have you tried a Seafoam treatment?
It may help that one cylinder, most cars run smoother afterwards
Follow the instructions on the can.