1998 S90 hesitates at low RPM

bloodrootfc · Post by **bloodrootfc** » 25 Apr 2014, 11:33

So I have the exact same problem (as well as system too rich code) on my 117000 mile S90. I have buttoned up all vaccuum leaks, new PCV, system, new MAF and O2 sensors. New secondary air injection valve.

After getting fed up with "try this" from 4 mechanics, I started doing my own diag with a Torque app and scan tool, and calling the shots. My mechanic works faster than I do, and he does a good job so I tell him how to spend his time, rather than do the repair myself.

What I have discovered from monitoring the intake maifold vaccuum with my scan tool is that I have a high vaccuum at idle (24-25 mm Hg as opposed to a healthy 22 mm). WOT test confirms this is not a compression issue, quite the opposite actually. The vaccuum drifts up and down 1 mm with the rpms at idle, which is indicative of the ecu trying to reconcile an improper fuel/air ratio. The only other explanation is:

THE TIMING IS TOO ADVANCED!

Timing is especially important at low speeds because the ecu is in a "closed loop". At higher speeds and high rpm the system is advanced by default as fuel gets dumped into the engine. What I am trying now is:

Make sure the timing belt is on correctly and hasn't slipped a notch.

Replace both knock sensors. These are tiny microphones that listen to the engine and are the most important electronic component of ignition timing control. They are constantly tuning the engine. Of course tuning used to be done by listening to the engine while manually twisting a regulator dial. Many people think that "tuning" refers to changing fluids and spark plugs, but really it is optimizing the timing. If the knock sensors are bad, your engine will be out of tune, and could cause hesitation, rough idle, poor fuel economy, and rich/lean codes.

Bottom line is if you have performance problems, it might be the timing. Will update after this is done and tested.

bloodrootfc · Post by **bloodrootfc** » 25 Apr 2014, 11:37

If it doesn't work my next thought is the fuel pressure regulator, and the transmission sensors/solenoids.

bloodrootfc · Post by **bloodrootfc** » 28 Apr 2014, 07:09

I should clarify, the problem I have is intermittent hesitation at low rpms. It only happens after coming to a complete stop, and only 5% of the time. I also get "System Too Rich" code 100% of the time whether there is a noticeable hesitation or not. The knock sensors I had installed made my engine hum nicely at idle, but didn't fix the problem. They checked the function of the ignition coils, fuel pressure, and transmission shifting. All good.

This morning I finally diagnosed a problem: I used my Torque app and scan tool to graph the front oxygen sensor through time. I have determined that the oxygen sensor is being affected by the application of the brakes. If I continuously pump the brakes in and out, I can make the front O2 stay lean (<0.1). After releasing the brakes, the system compensates by flipping too rich for the same period as the lean condition was detected, before returning to normal oscillation. Furthermore, I was able to log the exact same behavior when the hesitation occurred during a normal drive. I am having the brake booster system examined for leaks.

bloodrootfc · Post by **bloodrootfc** » 28 Apr 2014, 07:40

Result of the test: The brake booster was leaking heavily about midway through the range. At full brake or no brake, no vacuum leak. At mid-brake, a lot of vacuum leak. This is (probably) why the hesitation problem seemed to be most prevalent after coming off an off-ramp. Gradual braking from 70 to zero on a curved, inclined road means a lot of pumping the brakes to regulate speed. This resulted in a prolonged lean condition leading up to the stoplight, followed by a prolonged rich condition at the stoplight, which persisted for up to 20-30 seconds, meaning that if I tried to start up during the rich period, the car would choke on itself.

Will update when the brake booster is replaced whether the problem has been fixed.

Post by **rgk** » 29 Apr 2014, 06:19

This is an excellent write-up. What kind of scan tool are you using?

bloodrootfc · Post by **bloodrootfc** » 07 May 2014, 17:53

The scan tool I am using is OBDLink MX coupled with the Torque app on my Samsung Galaxy S3.

Update: The brake booster has been installed. I still get rich codes, as well as idle control system code. I have not had the hesitation again since installing the new brake booster. The idle control valve is new anyways. My 5th mechanic is now suspecting I have an intake manifold gasket leak.

Some general thoughts: Cars are made up of thousands of parts. After 100,000 miles, many things can go wrong at the same time. Some of the things are potentially related to a single symptom, while others are completely independent of each other and have their own symptoms. For example, recently I was overheating at an idle. Turned out the radiator fan relay had water in it. Replacing the relay fixed the overheating idle but did not fix my rich code CEL problem.

Replacing the brake booster seems to have addressed my hesitation problem but not CEL.

"Smoke tests" do not necessarily reveal all leaks. The gold standard is to spray propane around potential leak points while the engine is running and see how idle responds.

Still suspicious of the ECU, but as long as there are identifiable mechanical problems address them first (the latest suspicion is a faint hissing noise in the vicinity of intake manifold and it is the original gasket).

bloodrootfc · Post by **bloodrootfc** » 06 Jun 2014, 13:50

Update: Same problem persists. New spark plugs and ignition coils. No change.

EDIT: "Now thinking fuel system beginning with the purge valve." Was about to start replacing the purge valve but it seems to be functioning (switches on and off depending on engine load). Instead, my pending EVAP code set (still have pending too rich code) when I sprayed some WD40 on the hose leading from my new brake booster. The hose looks degraded. I can't believe someone didn't replace the brake booster hose when they replaced my booster. Seems fundamental. Argh.

Ethan Rode · Post by **Ethan Rode** » 07 Jun 2014, 03:30

It's not unusual for a technician to forego replacing those model specific vacuum hoses, if they appear serviceable. Spraying WD40 on any hose seems like a bad idea, since it causes rubber to swell. It is highly unlikely that the ECU is the problem, since it's digital, there's nothing to degrade. the sensors and secondary output devices, maybe. However, in my experience, the issues you describe are usually something simple. Maybe you could share your OBDII codes with us?

bloodrootfc · Post by **bloodrootfc** » 07 Jun 2014, 12:01

The codes are P0455 and P0172. The car hesitates from a stop (either no sound or a slight choking noise) but recovers after releasing gas pedal or pressing hard on gas pedal. There is also a bit of surging below 30 mph after the hesitation. Sometimes idles rough after driving a long time and then turning it off for 30 minutes - 1 hour. Have noticed a vibration of sorts during idle, rarr, rarr, rarr, and sometimes it feels like the gears are shifting hard on deceleration. When I leave it to idle for >20 minutes with A/C on, every ten minutes or so, the idle dips really low (like <600) and almost dies, but then comes revving back above normal before settling back into a normal idle.

Have also had the transmission fault flash once or twice before but not consistently. Only consistent thing is the CEL codes (both of them) are either pending or have set within 60 miles of clearing them, but most often within 10 miles.

Any thoughts would be appreciated.

bloodrootfc · Post by **bloodrootfc** » 07 Jun 2014, 12:13

I told them to do the ignition wires too but the part is not in stock. I really want to replace all the ignition components because I know how high voltages can fry parts.

EDIT: Now I understand what they mean by the part is not in stock. Because it is a coil-on-plug setup, the only wires running to the coil are from the main electrical harness. This harness can go bad due to heat from the engine.