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RENIX:Will a bad Knock Sensor cause an Open Loop?

my2monkeys

my2monkeys

NAXJA Member
Location
Mission Viejo, CA
My 89 4.0 keeps going in and out of closed loop as per mt2500 data.

I tested o2 sensor with an analog meter and it was switching when in closed loop(very briefly).

Also tested o2 heater and got 13.2 volts with engine running and on first turn of the key.

When I was down in that area of the o2 connector I noticed the Knock sensor connector was seperating from the body, mt2500 was showing 0 knocks at idle and rev'd to 1500rpms.

Will that keep it from closed loop mode?

Thanks, Colin
 
Renix will run closed loop with no problem with out a knock sensor. But I have never known for 100% sure if shorted Knock sensor wires would cause a problem. But I seriously doubt it.

What signs are you seeing that it is open loop?
 
cruiser54 said:
MT2500 Snap-On scan tool tells you.
Click to expand...

Tells you what, exactly? I was asking for any and all the data the scanner shows. Data man, data! LOL:wave1:

He also mentioned using an analog meter, so he may have data that would help from both.

What is the analog volt meter showing in open loop?

my2monkeys, did you test the O2 sensor heater itself with an ohm meter? The 2 heater wires, (disconnected from the harness) should read about 8 ohms.
 
cruiser54 said:
The scanner has an indicator showing open or closed loop.
Click to expand...
A volt meter can indicate that too, but a volt meter will also show lean or rich. Doesn't the MT2500 also show fuel trim?
 
Yes I tested the o2 heater and got 13 volts.

I have a few old o2 sensors lying around so I found one with a good heater element (7-9 ohms) and that got me back to closed loop.

But it still doesn't show any Knock values so your correct about closed loop with no Knock sensor.

I took some pics of scanner screen but I can't get them to upload thru SD card slot or USB port. I'll post them when I do.

The back story is: I swapped injectors for 703's 2 weeks ago and got a mt2500 after to monitor things.

My concern was my Long Term Fuel Trim was at 89 and Short Term never went above 90.

After some reading I disconnected the battery and reconnected and that put my LT FT at 128 but the ST FT is still down at 60-70 area.

I got a new o2 sensor on order so we'll see if that helps.

I have CA Smog due Feb. 2nd so I need to dial it in or maybe put the TomCo stock type injectors back in and see what the scanner says.
 
Kind of hard to see Some of the info. the MT2500 shows. .
picture.php
picture.php
picture.php

picture.php
 
The old school way to test a knock sensor is to get the engine warm and at an idle. Rap the block next to the sensor with a hammer a few times. You should see the rpm's drop. Now if you don't have the sensor connected, it can pick up stray electrical noise and the computer will think it has a ping problem and keep the timing retarded and hurt performance and mileage.

If you look at the wiring for the knock sensor you will notice that it has a shield. It is the only shielded wire other than the antenna on an XJ. The reason is that the knock sensor is actually a tuned frequency high impedance microphone. The high Z means that it can pick up noise easily.
 
Old Man, I pulled the Knock sensor housing off the body and messed it up.

Have JY a new part or do without it.

I bought a Bosch O2 sensor today and after start up it went into closed loop.

But after a few minutes I see the ST Fuel Trim going down from 30 to 10 to 0 for a second then Open Loop!

So Dang, it must be the 703's running it rich.

My ST FT have been low in the 60's but I can't compare them to anything because I had the 703's in when I got the MT2500.

Now I did re-loom the wires headed down to the O2 sensor (CTS,Knock,O2) I wonder if I shouldn't have put that Thermal covering made out of aluminum looking fiberglass stuff?

The O2 volts were going back and forth well on the new O2 until it slowly went south.

I wonder if I should take it out so not to foul it until I solve my problem?
 
You still have not told us if it is running rich or lean in open loop?

Have you checked the Vacuum line on the fuel pressure regulator to see if it is leaking gas, that bypasses the injectors? Do you know for sure that all the injectors seal tight, and do not leak? Beyond that I would go back to OEM flow rate injectors.
 
Sometimes it said it is Rich other is Lean but I think that's from faulty O2's that would show 0.00 volts or 4.98 at open loop.

See, I'm confused because when I had my 2 volt meters hooked up to an inline harness

(1-analog for needle swing and 1- digital for Heater 12volts) when the needle stopped

moving it stopped at 5volts. But now on the Mt2500 in open loop with the new O2 on the

scanner says 0.04volts.

I don't know I need to start at the beginning and quit guessing, I bet the O2 Heater

circuit will end up being the problem even though it was showing 13 volts.

Thanks for the input any suggestions are appreciated.
 
Well all I can say this late (I need some sleep), is keep track of it on paper, the conditions and data, or after a while you start second guessing your memory on details. Also keep track of "if the testing was at idle or say 2000 rpm"!!!! As that could make a huge difference!!!! At 2000 rpm steady the O2 heater is not needed!!! At idle it is.

I would keep the known, good working O2 sensor in there to speed up diagnostics. It will be fine.
 
Last edited:
UPDATE: Today on the way to work the motor would stay in Closed Loop on the freeway but as soon as I slowed down it went into Open Loop.

While in Closed Loop the ST FT stayed down in the 20's range.

After work I put in my stock type TomCo injectors replacing them dang Neon 703's.

Well, you guessed it, she fired right up to a stable idle and went straight into CLOSED LOOP.

That's on a cold motor and the ST FT were already up in the 80-90 range at idle.

I haven't driven it around yet(wife called me in for dinner) but I'm hoping all is well.

Data is telling me that at least my Renix doesn't favor the 703 injector and might not have

even known it was hurting me save for the MT2500.
 
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