Hi, All.

Can anyone on the board tell me (from 1st hand experience, preferably) the specific symptoms that a bad knock sensor causes at both idle and running speeds? The truck is a '87 F150 I6 fuel-injected 300CID (4.9L) with a 4-speed manual and 137,000 miles.

I've tried a search here on the FTE Forum using both "knock" and "knock sensor" as the keyword, but either get 1000's of hits or no relevant ones.

With all that's been done to this engine over the last year, it should be the best running I6 in the world, but it still has what appears to be an engine mis-fire at both idle and running speed after it warms up. You can see it in the shake of the gearshift ****, and feel it in the cab, too.

I won't go into the specific details of everything I've done to this engine, as the list would be 'way too long. I'll just say there's no code showing up on the ECM, compression is within +-5 psi on all cylinders (after a valve job), NO VACUUM LEAKS, all new or tested sensors (except the knock sensor), IAC cleaned, EACH injector physically removed and cleaner manually forced through it, new O2 sensor, EGR valve blocked off, new ignition module, wiring, plugs, coil, rotor and cap, etc. I've even had the ECM out to ensure there's not a bad connection in the firewall plug connector somewhere. (Now there's a fun job -- trying to get the weather-warped rectangular gasket reseated correctly in the firewall around the ECM!!).

Everyone says this I6 300 engine is "bullet-proof". That may indeed be the case from a purely mechanical viewpoint, but the methods used for controlling and diagnosing engine fuel and ignition control problems sure leave a LOT to be desired.

Thanks in advance for any help!

 

yeah its all crap.

If i had the first one built, back in the 12th century, I would be a happy man.

They should have kept it a purely tractor motor instead of puting all that snazzy crap like A/C and stuff in it. hehe

All the electronic garbage gets to me too....

I'm actually having the EXACT same problem as you right now, I did get an error code about the knock sensor * i think* but its hard to tell the codes with my crappy multimeter....

If i find the solution, i'll be sure to let you know.

 

Thanks, rehtor.

I think this problem (whatever it is) is a lot more common than people think it is. There's even a couple of guys who work at the local auto store that say they've been chasing the same "mis-fire problem" I'm having and can't find it either.

I'm coming to the conclusion that the knock sensor may have turned super sensitive over time, and is detecting "knock" when there isn't any. Once it detects something, it kicks in to retard the timing, the engine shakes a bit, and it detects THAT and retards the timing again. It gets itself into a self-sustaining loop of some kind. Just my theory right now.

 

My truck did the same thing until I swapped in 19lb injectors, now it's very smooth onthe highway but still has a miss at idle. If you get the truck scoped by a knowledgable shop they will be able to tell you if one cylinder is causing your problems and if so why. If it's multiple cylinders there will usually be a common thread causing each of them problems and it will be visible on the scope.

 

well, will the engine still run if the knock sensor is unplugged?

I need to get a chiltons...

 

Yes, the engine still runs. There appears to be no difference if the knock sensor is connected or not.

 

Mine has done it since it was driven new off the car lot.

 

To Silver Streak:

What do you mean by "a 19lb injector"? I've seen references to different "lb" injectors on other boards, but never a good definition of what that means. The writer just assumes all readers know what that means, I guess. Does it refer to how much gas one injector will pass in a certain period of time at the standard rated operating pressure, or what? If that's true, then 19lb would equal about 2.5 gallon, roughly, but over what time and at what pressure?.

Also, I prefer not taking the truck to a local shop, because I think they'd really try to rip me off. They have a reputation for doing that around here... I just got nailed for $300.00 to "clean the injectors" on my POS C**vy B**z*r", which I wish I had never bought. A scope analysis would probably wind up costing me more than the entire truck is worth!

In addition, I don't know if I'd trust their analysis of the situation even if they did say they found something. As an engineer, I'd like to solve this problem from a logic standpoint, but so far I've not been successful. But I'm not giving up yet!

To Tectron1:

Doesn't this shake just bug the livin' bee-crap out of you? I've seen F150 I6's run so smooth you could almost balance a dime on edge on the shift **** with the engine running. Nary a shake. So why do some do it & some don't? When they do, it makes you wonder if you're getting all the gas mileage and power out of it that you should be getting. That's my main concern. Have you ever mentioned this to the place where you bought the truck? If so, what's they say about it?

Thanks, all.

 

It just means that the injector flows 19 pounds of fuel per hour, or 3.17 gallons. That rating is for a delta P of 3 bars, or 43.5 psi, across the injector. Some manufacturers rate their injectors at slightly different pressures, so not all 19lb injectors are created equal. 3 bars is the industry standard, I don't know why some deviate from that.

Sorry for being obscure. I try not to include excessive information in my posts because there are those that want to crucify you for that. I don't think I've seen it happen here, but it is common on other 'boards.

I also have a degree in engineering and can understand your desire to find a logical solution. However, in the automotive world there are many problems that have no logical solution. I knew this before becoming a mechanic, but it is reinforced almost everyday. There is no logical reason for a bad head light bulb to cause the check engine light to come on, but it happens.

If you don't trust a shop to scope your truck there are some steps you can take at home. With the engine idling while it had the miss, install and inductive timing light on each plug wire one at a time. If the problem is ignition related you should see the light not flash whenever no spark is fired. If this phenomenon exists on all plug wires, put the timing light on the coil wire and check it there. If the coil spark is steady you have a problem betwen the coil wire and plug, if it drops at the same time the engine misses there is a problem with the coil or something in it's control system. If the ignition tests good you've got a fuel problem. You can artificially richen the mixture temporarily by unplugging the vacuum line on the MAP sensor or the fuel pressure regulator and plugging it. If it runs better with either of those seeing atmospheric pressure you have a lean mixture, if it does worse it is rich.

Have you checked the timing?

I know you said you have no vacuum leaks, but are you sure you ckecked everything? When I check for leaks on a vehicle I remove everything from the engine that it doesn't need to run and plug it. The evap canister, AIR system, brake booster, trans modulator line (not applicable to you), EGR, PCV valve, etc. The PCV system is actually a fair candidate to cause problems because it IS a vacuum leak. Even if the hose isn't leaking, the valve might be allowing too much crankcase vapor by and causing your problem. It seems like you are only having the problem when there is vacuum in the intake. That is a big red flag to me that there is either a vacuum leak under an unturned stone, or something that is controlled by vacuum is having a problem.

 

 

My dad was the one who purchased it off the lot origionally, It used to bug the crap out of both of us. The dealer mechanic said it just the nature of the beast on the fuel injected i6. Over time we discovered that other people had the same issue, that was almost 260k ago, it still does it. Since mine is an automatic I dont notice it quite as much.

The best mileage ive evr gotten out of this 94 auto 3.55 truck is 19mpg. My dad owned a 80 with a i6 4 speed and a 308 rear and it got consistantly 22+ mpg, so I dont think Im too far off on my mileage. Although it does make one wonder if they couldnt have made it better.

 

Traderjoe,

Have you turned up anything with the knock sensor? I have done the almost exact same thing with my 4.9 (even taking out the fuel injectors and cleaning them manually) but it still has the misfire. My engine was really junky on the inside of the intake manifold and I assume the head as well but I didn't take the engine down that far.

Anyway, did cleaning or replacing the knock sensor help?

 

i'm cahsing down the same problem.

So far diagnosis and help from FTE members has gotten me to either the map sensor or the fuel pressure regulator.

Check to see if it has any thing to do with vacuum. You're getting a lot of vacuum at idle, and at higher rpm low throttle situations. Those are the times i experience the mis-fire.

Try unplugging the fuel pressure regulator and see if that does anything. Silver streak suggested this to me and the mis fire dissapeared completely, so i've gotta check out that regulator.

 

BASSGUY31:

My son is driving the truck for a week, so I haven't been able to get to it to really dig into it very far again. However, it was suggested that the way to test the knock sensor is to unplug it. If the mis-fire goes away, then the sensor has become super sensitive. Otherwise, the problem is elsewhere. When I unplugged the sensor, I still had the problem. However, it could be that the connections inside the sensor to the piezzoelectric crystal itself could be open, therefore unplugging it would make no difference. I haven't had a chance to physically remove the sensor and check it with an ohmmeter. I have a lab-type oscilloscope that I should be able to view the output signal of the sensor on once I get it removed and can rap on it with a hammer.

We recently did a valve job on the engine, and there was some carbon buildup in the intake manifold but nothing that I would consider serious enough to cause the problem. Definitely not enough to significantly block any airflow, especially at idle. When I can get my hands on the truck again, I want to revisit the possibility of a vacuum leak, by disconnecting and plugging EVERY line that's not absolutely necessary for the engine to run. Some of those hoses, although they look and feel good from the outside, could be sucking air under a high vacuum condition.

The only other possibility I can think of at this time is an erratic output from the Hall-effect pickup in the base of the distributor. A long time ago, I had a car that had a loose shutter in the distributor on the pickup, and it would "rattle" thru the magnetic field. It was actually caused by too much "slop" in the distributor shaft, so the pickup would occasionally double-fire a plug. You couldn't see it with a timing light, but you could sure feel it in the engine performance. I had a terrible time figuring that one out!

I WILL solve this problem with this in-line six. It's obviously just going to take more time when I have access to the truck. I'll post back when I find the cause and cure.

Thanks for everybody's help.

 

You know, I have NOT seen ANY real reference(s) about replacing the TFI module (grey sensor attached to the base of the distributor).

FORD was KNOWN to have developed a DEFECTIVE type technology with TFI modules.

I bought a 87 EXP once (for $100) that had the symptoms as these trucks. I heard about Ford's defective TFIs, so I replaced it and BAM! It ran GREAT!! I sold the EXP for $900 (2) days later!

FORD's TFI Module problems were SO SEVERE, that there was a Class Action Lawsuit brought against FORD.

Here's the link to the TFI Module Litigation Settlement!

http://www.tfisettlement.com/