My 94 e320 surges at idle 85% of the time. My mechanic indicated the cause is no 12v from the cpu to the oxygen sensor heater and suggests we jump this from a 12v ingition source. Has anyone heard of this issue causing surging?

Prevalent cause of idle surging with this particular engine is a vacuum leak on one or more of the hoses.

You can possibly trace it yourself, but a mechanic who is familiar with the vacuum hose routing on the W124 can do this rather quickly.

I endured the surging for about 4 years before I had my mechanic trace the cause and fix it in about 15 minutes.

__________________
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

My mechanic, who is a pro and very good, has spend hours on this car and insists it's not a vac leak. Is he correct that the oxygen sensor needs to have 12 volts to the heater unit on it full time?

I previously owned a 1994 E320 station wagon. My wagon began surging at idle in 1998 (approx. 65K miles). I took the vehicle to the dealer (under Starmark), and the foreman suggested the engine computer was defective. Upon replacement, the car never had the problem again. You might want to find a used part or a DIY because I remember the repair would have been over $1000.

Not above 5K RPM [ WOT]

Have him do an 02 sensor test .
You can also check to see if the O2 is reading lean condition when the sig wire is to the ECU is disconnected.. if it does , then a vac leak is suspect.
Both the EGR and Purge valve can be leaking on a 104 HFM system and you will not hear/see it b/c the leak is internal to the vac system........... my first suspect after 02 test verification on a 104 would be purge valve

My mechanic said the cpu also but I could try and jumping the possitive heater wire to 12v ignition source. Has anyone tried this?

You could have a bad ground.
The 12v for 02 sensor is under the passenger rug.. You want to check for 12v at the bu/gn wire and battery neg.
If you have 12v using the bat neg, then try the same using the brown wire .. if nothing there , you have a bad ground,
.. which could fool one into thinking no power to heater circuit........

Also when surges come , twist fuses 6 and 7 and tap the OVP relay.. see if that straightens it out any.

You also want to ask if he took an amp reading of the 02 sensor heater.. the specs on the heater are 1.2-2.2 amps........... if the heater is shorted and the draw is more , then the ECU may be cutting power to the heater by default..............
Direct heater 12v power testing w/amp meter in series will tell him this.
He may have already done these test, but you do not mention any Resistance values , ect. , so hard to assume anything.

What harm done by trying the suggested 'jump from a 12v ignition source'; anyone?

regards,
mark

<>


My fear would be possible ECU damage, as the ignition source on HFM/SFI is direct power through the ignition module.
Which is why I recommend a simple Ohm /heater resistance and heater ground test or amp /draw test on heater circuit BEFORE any 12v aux feed is hooked up... and even then, you may want to fuse that feed , depending on where you get it from......

Test results:
Bu/gn wire: at fist nothing, then gradual from 6.5 to 14.2 volts and steady at 14.2
Brown wire: nothing to ground, so 12v going through heater

Resistive tests with ignition off and connectors seperated:
Engine computer, brown wire to ground 0 ohms
Engine computer, Blk wire to ground 96.2 ohms

Oxygen sensor grey to white no continuity (bad heater coil?)
Oxygen sendor white to white 2.7 ohms

So I guess I do have a bad oxygen sensor heater? Would that cause the idle surges?

Please suggest an economical replacement part. I want online and an site lists four 4-wire units, two from bosh and two from other vendors which are much cheaper.

Correction; brown wire test no 12v to ground, so voltage NOT going through heater, correct?

Hey sorry for all the mistyping; my kids are helping me and they don't spell very well!!!

You have some incorrect info for your test

The 02 sensor is as follows

White wires - heater coil.
Gray- neg 02 sig.
Blk- pos 02 sig.

The 3 test for heater are Resistance, short to ground, and current draw.
All these test are with connector unplugged.

test 1:
Ohms across white wires.. this is heater coil R test.
test 2:
Ohms across either white and gray. This is a heater short to ground test. There should be NO reading.
Test 3:
Bring 12v from battery to the white wires with an amp meter in series ..this is a heater current draw test. Looking for specs I mentioned in early post.

If all these test are OK, the heater circuit in the O2 sens. is OK
If any fail , replace the sensor.
These test only verify heater part of sensor and passing of these does not test the 02 sensors capacity to generate a sig to respond to rich/lean conditions, That is a different circuit of the sensor and a completely different test, using the gray/blk wire circuit... [ which I can post if you get the first test verified]

It is more than likely that your Indy has done these test, but I mention them only as insurance and for et al info...

This took awhile as I had to buy a meter that measured over 2amps.

Test 1 Ohms white wires: 2.4
Test 2 Ohms white to gray: NO reading
Test 3 white wires amps: 2.8A and after 15 seconds 1.6A

What's next, and thanks.

Can someone please help me with the next series of tests? What do I need to do?

Have you verified no 12v at heater circuit when car is in closed loop??