1994 Chevrolet S-10



April, 1, 2013 AT 5:22 AM

Where can I find impedence specs for sensors on my 4.3 L 1994 chevy S10? It is idling a bit uneven and wanted to check the impedance on crank positiion sensor, throttle position sensor, coolant temp sensor, o2 sensors etc to see if anything is out of range. No check engine light is on, idle drops then comes back up while sitting still in gear (automatic trans) Dont want to just replace sensors blindly, figured I could check impedance to see if anything is obviously out of range. Might have to get a factory issued manual to get these specs, they are pricey! Thank you very much, john


5 Answers



April, 1, 2013 AT 6:48 AM

You're going about this all wrong. Three-wire camshaft position sensors and crankshaft position sensors can't be measured because there's all kinds of circuitry inside. Two-wire sensors can read anywhere from a few ohms to over 1,000 ohms. No two will ever read exactly the same, and they won't cause your symptoms. Throttle position sensors will all read differently, and they could vary widely in resistance and it wouldn't matter. The wiper picks a point where it measures the percentage of total voltage applied. That won't change even when the resistance value is different. Also, the value of a TPS can't change. Oxygen sensors develop a tiny voltage. There is nothing you can measure, and if you did, I wouldn't know what to expect.

These values that can be measured may be listed in the service manual for reference but even there they give a pretty wide range of typical values. If a sensor did somehow change in resistance in a way that would affect engine operation, the Engine Computer would detect it, turn on the Check Engine light, and set a related diagnostic fault code. When you do get a code you will find almost always there's a break in the wire, a corroded terminal, or a total break inside the sensor that creates an open circuit.

For learning about engine sensors I developed a worksheet that required my students to measure the resistance of some sensors and the voltages on the signal wires. That was only to understand how they work, not to do that when looking for the cause of running problems. In fact, mechanics rarely test sensors individually until other testing points to one being defective.

You're right to not want to throw a lot of random parts at the problem. The computer learns their values and constantly compares them to what it expects to see from other sensors under various operating conditions. Relearning new sensor values doesn't always occur the first time the ignition switch is turned on. Specific conditions may need to be met and until then the new sensor will generate different signal voltages than what the old one would have. The computer will respond to those different voltages thinking they're coming from the old sensor.

One of the biggest problems GM has is with unequal flow rates from the injectors. Chrysler buys their injectors from Bosch in flow-matched sets. GM grabs a handful out of a big bin of injectors and stuffs them in with no regard to matching the flow rate. After many miles of wear and varnish buildup that mismatch shows up as one or two cylinders running lean. The oxygen sensor detects that lean condition and the computer responds by commanding more fuel, but ALL the cylinders get more fuel.

You also have to look at the ignition system as the cause of random misfires. On '96 and newer models the Engine Computer will detect which cylinder is misfiring and set a related fault code. On older models you have to determine that yourself.



April, 1, 2013 AT 7:09 AM

Very great information, thank you! I understand with these motors that the intake manifold gasket system from the factory is really crappy. Mine seems ok, but I read a lot of people replacing that and also their "spiders" which I am not sure what they are, something to do with the fuel injection system. Should I check for any intake leaks, and if so, is soapy water sprayed around the intake manifold a reasonable way to determine if I have an intake leak? I have changed the fuel filter and will be changing plugs, wires, cap and rotor just so I know they are good, then cleaning throttle body, running stuff to get rid of any moisture in the fuel and to clean the injectors, flushed coolant, changed oil and added lucas oil treatment to lube the timing chain and motor nicely. Anything else you might recommend? Just bought this old truck, it actually runs good, but the idle goes up and down a bit. I have had amazing luck with lucas oil additive, I notice an immediate increase in compression and a quiet valve train everytime I use it, do you think it is a good product? So if the o2 sensor goes out, I should get a check engine light on this old 1994? Can catalytic converters get clogged with age and could that be causing this up and down idle problem? Thank you again for your expert advise. John



April, 1, 2013 AT 7:13 AM

Finally- would a matched set of injectors help this problem out? I have read of kits like that for $250 or so that many say make their engine run much much better, perhaps that would be a reasonable strategy?



April, 1, 2013 AT 8:57 AM

First for the chemicals, we call those "mechanic in a can". My opinions were formed when I was an automotive student in the late '70s. At that time the instructors only believed in two things. One was a stop-leak additive for cooling systems and one was GM's "Top Engine Cleaner" that removed carbon buildup. Some people also had good luck with STP but that had to be added to a hot and running engine, otherwise it would just gel at the bottom of the oil pan and sit there. Since those days a lot of new products have become available and many of them do what they claim to do but I've never used them.

The injector cleaners you add to the gas tank are just concentrated versions of what is already added to gasoline. All refiners add something to keep injectors clean. They just advertise and promote them differently. The ones that really seem to get the best results are the soaps professionals use when they sell an injector cleaning service. They disable the fuel pump, then the engine runs for about 20 minutes on the stuff in the pressurized bottle. The biggest improvement is usually seen on high-mileage GM engines.

Varying idle speed is often caused by a vacuum leak. If the symptoms are present while you're under the hood you can pinch off hoses and if one affects it, work your way down to where it branches off, then follow one hose at a time to narrow down the location of the leak. The larger V-8s had a lot of trouble with leaking intake manifold gaskets but they typically leaked coolant out. I don't think that was common on the 4.3L. Also look closely at the fresh air tube between the mass air flow sensor and the throttle body. If any air sneaks in that isn't measured the computer won't command the appropriate amount of fuel to go with it. There can be a wide range of symptoms based on engine speed and acceleration when the leak occurs.

To check for a leaking intake manifold gasket spray water on them while the running engine is still cold. You don't need to add soap. You'll see and hear the water get sucked in if the leak is bad enough, and the engine speed will change. In rare cases the leak can be under the manifold where you can't see, hear, or spray it. You have to rely on interpreting readings on a scanner for that. With small vacuum leaks the oxygen sensor will detect the unburned oxygen and add more fuel in an attempt to correct the mixture. If you view the short and long-term fuel trim numbers they will be high positive numbers meaning the computer is adding fuel beyond the factory-programmed values.

You can also use a propane torch, (no flame), to flood suspected areas with fuel. If there's a vacuum leak that fuel will displace the air and you'll see the scanner show the exhaust is staying rich longer. The oxygen sensor doesn't really detect unburned fuel. It just fails to detect excessive unburned oxygen. Eventually the computer will reduce the amount of fuel from the injectors.

A misfiring cylinder will also result in excessive unburned oxygen being detected. If the problem is spark-related, unburned fuel and air go into the exhaust where only the oxygen is detected as a lean condition. The computer doesn't know it's due to just one cylinder so it commands more fuel to all of them. With a restricted injector you will have just excessive air and again the unburned oxygen will be detected. With a spark problem, no matter how much extra fuel the computer adds there will always be that unburned oxygen still being detected. The short-term fuel trim numbers will be real high positive.

Much of my information on injector trouble comes from a very high-level class put on at my community college by Jim Linder. He owns a company in Indianapolis that specializes in rebuilding injectors. If you do an internet search you can read about the most common problems they run into. They solve the most complaints on GM products by just selling matched sets of injectors but there are many other things they tell to look for.



April, 1, 2013 AT 9:51 PM

Awesome, thanks for all the info. I have seen a lot of internet ads from forums regarding new matched injectors and improved intake manifold gasket systems. The 4.3 had a major tech bulletin on crappy factory intake manifold gasket systems. Mine is ok, I will search for vacuum leaks and see how it goes. What the lucas additive does is it really clings to moving metal, so it does a fabulous job of lubing timing chains and valve train, it really does reduce valve clatter noise and also clings to cylinder walls thereby increasing compression- it is for sure noticeable. It has been around for many many years. I suppose some sea foam added to fuel might help to slowly reduce carbon deposits on upper cylinder walls, I know it melts carbon, I have noticed that one must put only recommended amt into fuel, too much will melt carbon too fast and motor will smoke burning that liberated carbon. Thanks again, john

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