• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

760 90' 760T Stalling, backfiring, bad idle

N

nickvolvo746

Member
Joined
Nov 13, 2017
Location
New Jersey
After a couple hours checking around other posts I think I still need a helping hand from one of you knowledgable folks.

In my 1990 760 Turbo with b230ft:
I have been having trouble with idle hunting and being generally a bit low, and recently it has been stalling frequently while driving and coming to a stop, even after engine is warm. At first I was able to keep it moving by messing with the throttle but it has gotten worse. It has also been backfiring a bit. Rpms bounce around alot when the engine starts to cut out. I am no longer driving it as it is happening so frequently and I dont want to damage anything. Im thinking the golden clue here might be that the rpms are bouncing around so much, but im unsure what to do with this clue and havent found anything enlightening yet. Thanks in advance if youre taking the time to read this, you guys rock!

Check Engine light threw codes for:
-bad AMM,
-faulty signal from coolant temp sensor
-02 sensor
-fuel trim too lead or too rich

Pretty positive the 02 sensor is fine as it is very new, and the AMM is also good. Wiring to both looks good, nothing corroded or shoddy looking. Definitely smells a bit gassy sometimes.
 
nickvolvo746 said:
Ive used it on my 740 recently as its AMM had stopped working and i have tried disconnecting it but to avail.
Click to expand...

If you disconnect and get the same running condition, the AMM is likely toast (dead). Failed crank position sensor = no running.

Vacuum readings whilst it is idling poorly?

Checked your IAC function?

You could have multiple issues in play here..... tricky to diagnose online without more data.

You need air/fuel properly metered together, and a properly TIMED spark for success. A timing light will tell you something as well, do you own one?
 
DET17 said:
If you disconnect and get the same running condition, the AMM is likely toast (dead). Failed crank position sensor = no running.

Vacuum readings whilst it is idling poorly?

Checked your IAC function?

You could have multiple issues in play here..... tricky to diagnose online without more data.

You need air/fuel properly metered together, and a properly TIMED spark for success. A timing light will tell you something as well, do you own one?
Click to expand...


I do not own a timing light but i found a cheap one at autozone for 20 bucks so maybe ill go grab one today. Where would i aim it at? As for IAC I admit I have no clue how to test that even after reading some stuff on here about it. Seems like IAC would really only have affect at idle? or is that wrong...and thanks for your response!
 
You said backfire in the thread title. Do you mean backfire or misfire - there is a huge difference. Backfire is an explosion back through the intake system. Back fire is typically caused by ignition timing that is really out; but, could also be a valve timing issue (really unlikely). Misfire is a failure of the fuel mixture to ignite correctly and is usually accompanied by the engine shaking and popping noise out the tail pipe. Lots of potential causes for misfire.

I agree with DET17, if you can disconnect the MAF and it has no effect on engine operation, chances are that you have a MAF problem (or broken wires to the MAF?). The B230FT will run with the MAF disconnected; but, really badly.

In terms of checking the ignition timing, I recall that there should be a pointer on the front timing cover and marks on the front pulley / harmonic damper indicating degrees before and after TDC. The marks on the pulley will probably be covered in crud so you may need to clean off the pulley to find them. With a $20 timing light you will likely have to dab the marks with a little white paint to see them when the engine is operating.

If the engine does not respond to smooth increases in throttle position (the car will not accelerate smoothly) then that problem is unlikely to be caused by a IAV problem. IAV problems generally show up at idle, typically the inability to hold an idle speed.

Pull your spark plugs out and have a look at the color of the insulator tips. That will give you some indication as to whether the engine is running grossly lean or rich. On my 1987 B230FT, the car started driving really badly after the fuel pressure regulator went way out of spec causing the engine to run so rich that it would not idle. You need access to a fuel pressure gauge to check that. However, lean / rich operation can also be caused by a failed O2 sensor and the MAF failure.
 
Last edited:
142 guy said:
You said backfire in the thread title. Do you mean backfire or misfire - there is a huge difference. Backfire is an explosion back through the intake system. Back fire is typically caused by ignition timing that is really out; but, could also be a valve timing issue (really unlikely). Misfire is a failure of the fuel mixture to ignite correctly and is usually accompanied by the engine shaking and popping noise out the tail pipe. Lots of potential causes for misfire.

I agree with DET17, if you can disconnect the MAF and it has no effect on engine operation, chances are that you have a MAF problem (or broken wires to the MAF?). The B230FT will run with the MAF disconnected; but, really badly.

In terms of checking the ignition timing, I recall that there should be a pointer on the front timing cover and marks on the front pulley / harmonic damper indicating degrees before and after TDC. The marks on the pulley will probably be covered in crud so you may need to clean off the pulley to find them. With a $20 timing light you will likely have to dab the marks with a little white paint to see them when the engine is operating.

If the engine does not respond to smooth increases in throttle position (the car will not accelerate smoothly) then that problem is unlikely to be caused by a IAV problem. IAV problems generally show up at idle, typically the inability to hold an idle speed.

Pull your spark plugs out and have a look at the color of the insulator tips. That will give you some indication as to whether the engine is running grossly lean or rich. On my 1987 B230FT, the car started driving really badly after the fuel pressure regulator went way out of spec causing the engine to run so rich that it would not idle. You need access to a fuel pressure gauge to check that. However, lean / rich operation can also be caused by a failed O2 sensor and the MAF failure.
Click to expand...


Thanks again for the response, this is a big help! About to triple check the
MAF and pull spark plugs. I don't think its an IAV problem as it does respond well to small changes in throttle, except of course when it is missfiring(which it definitely is, after reading your description:popping out the tailpipe and shaking). Can I run the same test with the 02 sensor that I do with the MAF? and by that I mean unplug it and see how it reacts? or will it just not work at all? My apologies for any dumb questions, Im new to this whole working on my own car thing and unfortunately I dont have the repair manual, I should probably get on that.
 
On my 1987, the O2 sensor was a single wire (non heated) sensor. The quick 'high level' test requires the use of a high impedance voltmeter or a scope to detect oscillations in the O2 signal. A digital voltmeter may work if it has a very high update rate. Cheap digital voltmeters will not work because the update rate is typically too slow and you will just see the reading jumping around. The less expensive digital voltmeters should work if you do the test where you induce a lean or rich running condition because the O2 signal is not oscillating.

Go to this link - it will give you the details on how to check the O2 sensor (scroll towards the bottom of the page):

https://www.brickboard.com/FAQ/700-900/EngineFuelinjection.htm#Oxygen_Sensor_Diagnosis
 
142 guy said:
On my 1987, the O2 sensor was a single wire (non heated) sensor. The quick 'high level' test requires the use of a high impedance voltmeter or a scope to detect oscillations in the O2 signal. A digital voltmeter may work if it has a very high update rate. Cheap digital voltmeters will not work because the update rate is typically too slow and you will just see the reading jumping around. The less expensive digital voltmeters should work if you do the test where you induce a lean or rich running condition because the O2 signal is not oscillating.

Go to this link - it will give you the details on how to check the O2 sensor (scroll towards the bottom of the page):

https://www.brickboard.com/FAQ/700-900/EngineFuelinjection.htm#Oxygen_Sensor_Diagnosis
Click to expand...

Mine runs with a 3 wire sensor, I bought a brand new one(reluctantly), threw it in, reset the ecu, cleared the codes and also seafoamed it. Idling much better and CEL is now off, if im not out of the woods yet i at least see more of a path, thanks for the response!
 
You must log in or register to reply here.
Back
Top