I was wondering if anybody had some insight on knock sensors. I've got a 1991 240 NA with a Bosch FI system. Still getting horrid mileage, about 14mpg's last I checked. I’ve been trying to test my knock sensor and found that there really is no test I can do as I don't have an oscilloscope. I know at the FCU I should be getting about .7 volts, however I am getting .98 volts. Now does that mean I have a bad sensor or a bad FCU?

resistance across the knock sensor is infinite and the voltage it produces is jumping around all over the place, from .03 mv - 300.0 mv and tapping seems to have no effect.

I don’t know if this is a valid test however. can anyone verify?

 

This might apply. My '93 940 Turbo was getting pretty bad mileage, yet the knock sensor was fairly new. I had been using reg gas and when I switched to premium the mileage went up. Any knocking due to low octane gas will retard timing and mileage will suffer. Also, wondering if their fuel addivitive might make it even better?

 

for sure, the turbos are tuned for premium, at least 91 octane. you can burn regular in them if you drive like a little old lady, but if you drive that way, WTF did you get a turbo for??

 

Excellent question! Why? Because it was there. It was "buy the turbo" or climb Mount Everest.

 

Ive ran tanks of 93 octane and there has been no change, because I also used to have some turbo cars, and the same thing was going on with me.

so good tip, however, no fix...

 

Boy, without any vacuum leaks, the only thing I can guess at is maybe the O2 sensor? What else controls mixture?

 

vacuum and intake air leaks can be hard to find - did you inspect the air tubes and intercooler etc? You may need to do a smoke test to be sure. The front O2 sensor controls mixture but fuel pressure plays a role so that brings in the MAF, MAP sensor, fuel pressure regulator etc into play.

 

Since I started taking fuel I've done the following:

Replaced plugs, wires, rotor and cap
Replaced air filter
Replaced exhaust gaskets
Tested the coil - good
Tested the MAF and replaced it with a known good unit
Checked for air leaks, found and fixed a pinhole in the intake hose
Changed timing belt
Checked fuel pressure - good
Checked o2 sensor and it seems to work in that it varies voltage within spec
Cleaned all grounds
Replaced PCV system
Replaced all vac check valves and old vac lines
Tested fuel injectors within spec
Cleaned throttle body and IAC valve
Adjusted TPS and kickdown cable
Ensured brakes are not binding

The only concerns are the possibility of a stuck injector
A lazy but technically good O2 sensor and
A bad knock sensor

The exhaust system is strange, because when I got the car it was missing the manifold flange gasket therefore it was leaking like crazy. Also my CAT is hollow, and I'm not sure if that's from an excessively rich life or an owner who thought it would be cool to gut it.

 

I forgot to ask the obvious - Is your tail pipe sooty or dry? If it's wet and sooty your engine's running too rich. If it's dry your transmission may be slipping.

 

Lots of dark soot, but pretty dry (its raining today si I will need to double check)
The pipe angles down, and if I idle for a while it'll leave a soot spot on the ground.

As for the transmission, it doesn't feel like its slipping...Ive had one slip before but Ive also never felt a bad transmission on a RWD car...

 

Ok. When I first start my 940 I'll get steam coming out of the exhaust and a wet spot under the tail pipe. Common in all cars, some more than others. Once it's warm - no spots. If yours continues to leave a spot(and the pipe stays dark) it's gas not water. Meaning it's pumping more than it can burn. What about the FPR(fuel pressure regulator)? I don't see it on your list. If it's not functioning right 2 things can happen. If it leaks or doesn't hold vacuum it can send too much gas back to the tank and have firing problems or not send enough back, which would keep it in the fuel rail and manifold and force your engine to get more gas than it needs. What do the plugs look like? You can test injector firing patterns with noid light rentals from Autozone or Oreilys. They could be spraying enough but how's the firing sequence? How's the engine's driveability? Smooth? Misses?
http://easyautodiagnostics.com/misc-...-tool-review-1
And one other simple thought = what about the temp sensor. Also not on your list. If your engine thinks it's cold it will override everything and feed extra fuel.

 

Ah I forgot that one, I did test the temp sensor from the ECU side. I homed it out when cold and then checked the temp of the coolant when hot and tested the resistance again, all within spec +or- 5%

as for the FPR, its not leaking, and the vac line is dry, but I don't know if its "Actually" working. When I remove the vac line there is definitely no change in the cars behavior.

I didn't think about it but I have never tested the order of the injectors, I simply assume they are correct. They all pass functionality tests, they have the right resistance, and OBD can click them on and off. I have never removed them form the block however so I don't know about their spray pattern and if they are stuck or not.

My plugs look good, slight soot, but they are also pretty new. The old ones were whitened from being simply too old (best guess)


The engine will at times start instantly, other times you can hear it churn up to 12 times before it will kick over, then will lope until you rev it a little. The engine runs a little hard and you can see it shake every few seconds (like its running fine then suddenly misses once)

Thanks for your help by the way!

 

Glad to help, I've been in your shoes. My gut feeling thinks your injectors are not the problem. If they we're I think you'd have more profound driveability issues. If, at times, it's running fine then suddenly misses that sounds like an ignition(firing) problem. I think it would take a ton of gas being dumped instantly to foul a plug and not fire (and it would foul all the plugs and probably shut down). If you had 1 bad injector I think the misfires would be more consistent. And you still wouldn't have long starts. I can pull 1 spark plug wire completely and my car will still start. So, if your cap and rotor are new and clean and you don't have a distributor seal oil leak which would cause an internal misfire in the distributor I'd take a real close look at your crank sensor and it's wiring. When I first got my car I had similar, not consistent, problems - quick starts then long starts, sooty tailpipe(and low mileage) intermittent misses. etc. I looked at the crank sensor twice and it looked ok. Then one day it wouldn't start at all and had to tow it home. Pulled the sensor and the underside of the wiring showed some chafing and looking closer I saw the internal braided section(looks like TV cable) had gaps in it. Replaced it with a $5.00 special from the local Pick n Pull and problem solved - consistent quick starts, dry and clean tail pipe and no misfires. Worth a try.

 

Checked the crank sensor and everything looked great with it when it came to the shielding, the sensor head and how it tested...Im going to replace it for peace of mind and who knows it might be the case after all.

I found a hole in the exhaust pipe again, about an inch of the flange gasket before the CAT is missing and so its a pretty significant leak. Im going to weld up a new system, but that might take some time.

 

By the way what is your gas mileage . Our 89 740 N/A with 360K miles gets maybe 20-22 mpg . New it got 24 .

 

its between 16 and 17 mpg on regular or premium

Ive tried to do things like verify the F/A ratio, but no matter how much additional air I introduce the car keeps on running fine. What this would indicate to me is that its fuel trim is as high as it can go, because you shouldn't be able to pull off all the vac lines and get no discernible change in running operation, that is unless its got more than enough fuel.

 

Yes, you shouldn't be able to pull vacuum lines and get no change. You may want to try this with the FPR. Pull the rubber return hose to the tank and stick it in a can or jar and have someone start the engine. If it's working properly, at idle you should have a steady flow of gas into the can(careful, it will fill up fast). If not, it's keeping it in the fuel rail. And here's the basics on knock sensors: https://www.volvoclub.org.uk/faq/Eng...ngaKnockSensor

 

I'd also consider checking the engine coolant temp sensor and the MAP (air pressure) or altitude sensor - not sure which your model would have. While its possible the knock sensor may be bad, I think ECT is a more common failure point. Also should you pull the injectors to check for flow or for a cleaning, consider replacing the o rings.