I have a 1994 Oldsmobile cutlass Ciera with 125019 miles with the 3.1L engine. The problem is I’ve replaced the camshaft position sensor 3 times and it does the same thing, when it’s plugged in it runs like it’s missing a cylinder and when it’s unplugged it runs good enough to drive. A number of other sensors have already been replaced and the computer has been replaced. I replaced the sensor in the first place because it was revving itself up and down and the sensor seemed to fix it but caused another problem.
I don’t have a obd1 reader.
You can get an OBD1 code reader pretty easily and inexpensively, this is what they look like:
Of course OBD1 codes are very limited but it's probably worth checking.
I did get to see the chain recently and it looked a little loose. How tight should it be?
There should not be much slop and little or no lost motion between the crank and camshaft.
Also I got fed up with the carquest brand so the last sensor I put in was a walker brand. I wanted to be extra sure it wasn’t a faulty sensor.
For best results OEM sensors need to be installed. Aftermarket sensors frequently do not work properly. Additionally the quality of even OEM parts can be terrible and aftermarket parts horrendous. Car Wizard goes into this in a recent video:
https://www.youtube.com/watch?v=Falkhbl14m0&t=375s
You don't need to go to a dealer for OEM parts. They are available online at a discount. (For example rockauto.com sells an AC-Delco cam sensor for that car for under $50.) However you should do a thorough diagnosis before replacing parts.
Haynes manuals are not always that great. You can find a free online repair manual here that is taken from the factory shop manual:
https://charm.li/Oldsmobile/1994/Cutlass%20Ciera%20V6-3100%203.1L%20MFI%20VIN%20M/
You will also find technical service bulletins and recall information at the above link.
Also realize that if using that 31-year-old car as a daily driver that's really pushing the envelope.
Here's a short video where timing chain wear/slop on a pushrod engine is discussed and demonstrated:
timing chain could be worn out. reluctor ring could be dirty.
Start with simple things before throwing sensors and computers at a problem.
@imperator I’ve looked through the hole the sensor is in and what it was pointing at was smooth and shiny. And if I unplug the sensor it runs better so I wouldn’t think it would be the chain. Is there a way to test the reluctor ring? This is my daily driver and I can’t afford a mechanic. The engine is running worse the longer the sensor is unplugged.
if the chain is worn, then camshaft sensor gives bad data. When you unplug camshaft sensor, the computer switches to using crankshaft data as a substitute. If you can't afford a mechanic, then you can't afford to keep replacing sensors and computer that work fine.
I’ve looked through the hole the sensor is in and what it was pointing at was smooth and shiny. And if I unplug the sensor it runs better so I wouldn’t think it would be the chain.
In what way does that rule out a timing chain problem?
Did you test your cam sensor before replacement to see if it was actually bad? Were the replacements OEM parts? Did you check for any OBD1 codes?
the car is worth fixing because the rest of it is all brand new( I’ve been fixing it over the years)
If this car is in really excellent condition and you intend on keeping it on the road as long as possible you might want to look for a deal on a used Tech2, which is the factory scan tool for GM cars from the early 1990s through the mid-2010s or so. This would give you access to all electronic diagnostics and settings available to the dealer at the time that car was new. You might be able to find a used Tech2 at a shop that is closing or from someone who bought one and no longer needs it. You would want one that includes the GM software as well as connectors for pre-OBD2 models. (I have a Chinese-made Tech2 clone for my own use that, while not as well made as the original GM item, works well and is fine for occasional use.)
mountain joe guy, no the computer was not a waste of money because now I have an extra computer
chances are you will never need the extra computer, and it didn't fix the problem. The point which you missed, is that you will spend less money and time by troubleshooting instead of guessing. Throwing parts at a problem is like rolling dice and hoping you win. Buying three sensors instead of checking to see if the sensor is really the problem, is the same superstition as blowing your paycheck at the casino because surely if you lose two games then your luck must turn on the third.
I came on here for help to fix my car which is what this website is for
The website is for car advice, and sometimes the correct advice isn't what people want to hear. Scotty tells people to sell their cars all the time.
Spending good money after bad can be the wrong choice , and many people foolishly go into ruin thinking "just another $2k and I'll be out of the woods". If the rest of the car is in good condition as you say (mechanically, not just cosmetically) and not a money pit, then carry on. Of course we'll try to help, but there's only so much you can do over the internet with limited information and tools.
I can’t reply to just one person it won’t let me.
You can reply to one person with the "add a comment" button or just mentioning their name like you did.
Here are some resources you can use to diagnose the issue the way a professional mechanic would:
- Basics of Crankshaft & Camshaft Position Sensors
https://www.aa1car.com/library/crank_sensors.htm - Timing Chains, Gears & Belts
https://www.aa1car.com/library/ar594.htm - The Basics Of Crank And Cam Sensors And How To Test Them
https://easyautodiagnostics.com/misc-index/ckp-cmp-sensor-basics - GM 3.1L engine diagnostics
https://easyautodiagnostics.com/gm/3100 - Factory service manual - Flow of diagnosis
https://charm.li/Oldsmobile/1994/Cutlass%20Ciera%20V6-3100%203.1L%20MFI%20VIN%20M/Repair%20and%20Diagnosis/#Powertrain%20Management/Computers%20and%20Control%20Systems/Testing%20and%20Inspection/ - Bench testing a camshaft position sensor (for trucks, but procedure should be similar)
https://easyautodiagnostics.com/gm/4800-5300-6000/cmp-sensor-bench-tests
I’ve looked through the hole the sensor is in and what it was pointing at was smooth and shiny. And if I unplug the sensor it runs better so I wouldn’t think it would be the chain.
You can't tell if the timing chain is off by thousandths of an inch or less by observing it visually. If it were "off" enough to notice with your eyes, the engine probably wouldn't run at all.
The timing chain doesn’t wear out evenly. Every time you shut the car off and start it back up, the timing chain starts wearing out until the oil pump builds up sufficient pressure. Over thousands of start-up cycles, random chance says that certain links in the chain will wear out more than others, eventually leading to uneven firing.
When you don't have the camshaft position sensor installed, the crankshaft position sensor takes over the engine's firing order. The PCM "knows" more or less when to fire the spark plugs because it's been programmed into it. The camshaft position sensor fine tunes the firing cycle so it's more precise. When the timing chain is stretched, the fine tuning effect is lost.
@justin-shepherd The crankshaft position (sensor has 24x more resolution) is always used for ignition timing on this vehicle, via the ignition control module. The camshaft position is only used for fuel injection timing, because it needs to track the intake valve opening very closely. On this vehicle anyway.
Makes sense, I kept thinking about it after I made the post. As your RPMs increase, the ignition timing must be more advanced in order for combustion to be completed before the exhaust valve opens. I'd bet if he revvs the engine without the cam position sensor, the engine will start running oddly.
@justin-shepherd well again, ignition timing comes from the ICM which only measures crankshaft, so camshaft isn't used. Combustion needs to track piston position closely to maximize power stroke. Timing is advanced, because flame front speed is not direct relationship with engine RPM. Exhaust probably isn't all that critical.
I see what you mean now, I became really confused for a second. Fuel injector pulses are determined by the crankshaft position sensor, then? That will make it run rough if there's enough slack in the chain.
@justin-shepherd No. Like I said fuel injection uses camshaft timing (per the factory manual). The injector needs to spray right as the intake valve opens to catch the pulse of air for good mixture right. If the computer doesn't get a camshaft signal, it switches to using the crankshaft signal as a failsafe.
@justin-shepherd my thinking was this: a worn timing chain causes engines to run poorly (valves and pistons aren't coordinated optimally). Unplugging the camshaft sensor somehow compensated by shifting fuel timing. But as I type this out now, it sounds stupid. I don't think you can "fix" bad valve timing with the fuel. It's more likely the sensor is giving bad data. but why? Did the shaft twist? Did the tone ring slip? Is the connector shorted out? It needs a closer look with signal analysis. I'm just not sure a 30yo car is worth investing the time.
I don’t have a obd1 reader. For my year of car (1994) even though it’s obd1 I still need a scanner. That’s what the Haynes repair manual says. As for the confusion about the chain I should mention I don’t know a lot about cars just a little. As for the computer I replaced that because there were other things that were going wacky that the computer all ran. Like the locks acting funny and the cruise control not working. But the problem at hand is the engine. I did get to see the chain recently and it looked a little loose. How tight should it be? Should I be able to wiggle it? All I got is a dirt driveway to work in so I don’t want to replace it if I don’t have to. Also I got fed up with the carquest brand so the last sensor I put in was a walker brand. I wanted to be extra sure it wasn’t a faulty sensor.
As for the computer I replaced that because there were other things that were going wacky ... Like the locks acting funny and the cruise control not working.
Did that fix it?
No the computer didn’t fix it. I’m guessing the other problems are just coincidence. The timing chain hangs about a quarter inch away from the guide and I can push it to touch the guide. Is that too loose? I guess the chain would have to be tighter than that for the sensor to have a proper reading? If I need a chain what brand would you recommend?
@jrock3 Look at the video I posted, that has an example of a chain being too loose on a GM engine at only 95K miles.
@jrock3 "No the computer didn’t fix it. " so it was a waste of money, and you just proved my point again about the parts canon approach.
mountain joe guy, no the computer was not a waste of money because now I have an extra computer in case the 31 year old computer should go out and the car is worth fixing because the rest of it is all brand new( I’ve been fixing it over the years) I came on here for help to fix my car which is what this website is for and I think I got it, if not I’ll look elsewhere. Also I did not see all the other replies before I made my post and I can’t reply to just one person it won’t let me.