Mechanics

FAULT CODE FOR THE CAMSHAFT POSITION SENSOR

1998 Pontiac Sunfire • 140,000 miles

Car starts fine and runs great for about the first 5 to 20 minutes. After that when I come up to stop light and the car is idling it starts running rough, the check engine light comes on and I have to pump the throttle until RPMs come up to about 2000 RPMs, then it runs fine until I get the next stop light. At point, The more I drive the worse it gets, to the point that it will stall out, but when I come to the stop sign, if I put it in neutral and shut the engine off, then start it up when the light turns green it runs fine until I get to the next stop light then problem starts all over again. If I let the car set for 10 minutes or so it will run good for another 15 or 20 minutes. Also I get a crankshaft position sensor fault. I replaced both the crank sensor and the spark control unit behind the coil packs. Still have the same problem and the faults showing up. I don't want keep changing out parts until I stumble on to the fix. I am thinking the problem is in ECM, but I am not sure. Steven.
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StevenDowell
September 20, 2013.



Scan for codes and check fuel pressure first with a gauge, auto parts rent eht gauges. Could be a connection problem as well.

Hmac300
Sep 20, 2013.
I have already check all of the connections for lose or corrosion or for a power loss between connections. I am not finding any fault codes that are related the fuel system. As I stated in my original, there are only 2 fault codes that are showing up, 1 is P0335 which is the crankshaft position sensor circuit and the other one indicates there is problem with the camshaft position sensor, I am sorry I do not remember the exact code for this sensor. As I already stated I have already replaced both units that are related to the crankshaft position sensor. Please explain why you might think that it is fuel system problem. Thank You, Steven

Tiny
StevenDowell
Sep 20, 2013.
If it doesn't have enough fuel pressure it won't run very good or long at all. Your crank and cam sensor connections could be bad as well.

Hmac300
Sep 20, 2013.
I am sorry, but when I submitted my first question it should have read 15 to 20 minutes not 5 to 20 minutes. I check for a power loss between the connections an found now problems at those part of electrical connections. If I am having a problem with fuel pressure would it not happen a lot sooner and would that not also be detected by the ECM. I would like to contribute a little more if we can come up with some that makes a little since, but I do not know how. Thank You, Steven

Tiny
StevenDowell
Sep 20, 2013.
Something I need to add as long as I am driving without stopping for a stop sign There is never a problem. Steven

Tiny
StevenDowell
Sep 20, 2013.
Low fuel pressure is hardly ever detected by the ecm and a bad fuel pump could run 15-20 minutes before problems. You can also try to clean the throttle plate on both sides with choke cleaner as well as iac hole while you are there. You have to check fuel pressure with a gauge, there is no other way to do it. If it's a few pounds below minimum it will cause problem.S. Fuel pressure should be 41-47 psi. But try the throttle plate thing first.

Hmac300
Sep 20, 2013.
Hi guys. Excuse me for butting in but I have a different thought. I think hmac300 is on the right track with fuel pressure but with a different cause. I had a very similar problem about ten years ago with my '88 Grand Caravan caused by a collapsing pickup screen in the gas tank. On this vehicle it only acted up on the very hottest few days of summer, (don't know why), then six months later it really caused a lot of trouble in the middle of winter. That's when my students were able to find it.

One of the clues that I didn't initially understand was the stalling only occurred during coasting down from highway speeds. It ran fine while maintaining highway speeds. The cause wasn't fuel pressure directly; it was fuel volume. Fuel volume moved by the pump goes way up during coasting when the engine actually uses the least amount. During coasting, intake manifold vacuum goes way up. That is one of the forces pulling fuel out of the injectors. The other one is fuel pressure pushing the fuel out. To prevent a rich coast-down condition, there is a vacuum hose attached to the fuel pressure regulator. When vacuum goes up, fuel pressure goes down. The net difference in forces acting on the fuel stays constant that way.

The problem is that by relaxing the spring in the fuel pressure regulator, it is easier for the fuel to flow through the return line and back into the tank. THAT is where fuel volume goes way up even though the amount of fuel going into the engine goes down. With the plugged pickup screen, that higher volume of fuel can't get through, and since it's actually bleeding off through the regulator, pressure drops and causes rough running or stalling.

I've been running my van for the last year with a pressure gauge hooked to the radio antenna because I had a similar situation, but only when pulling a really large tandem axle enclosed trailer that's bigger than the van. Coincidentally, I only drag it around twice a year in July when it's hot again. It turned out to be another plugged pickup screen, but while watching how it acted, I found that the normal fuel pressure of 45 - 50 psi could drop to as low as 20 psi and the engine still ran fine with no symptoms. It was at 15 psi that it started to sputter. Most GM engines are not nearly so forgiving. Some of them will not start or they'll stall when the pressure is just five pounds low.

I had this same problem with two carbureted cars years ago. The time frame you described was exactly the same. They'd run fine for about 20 minutes at highway speed, then they'd feel like they were running out of gas. I learned to just sit on the side of the road for five minutes to give the screen a chance to stretch back out, then I could make it about another five miles before it acted up again.

Now that I understand what was happening on my van, I suspect I could have verified it by disconnecting the vacuum hose from the pressure regulator and plugging it. That would result in too much fuel pressure and black smoke from the tail pipe, but it shouldn't stall.

Normally the engine will stall if the crankshaft position sensor or camshaft position sensor fails, and they often fail by becoming heat-sensitive, then work again when they cool down for a half hour. One possible explanation for the fault codes, and the rough running instead of outright stalling, is that the engine is going too slowly due to the fuel problem and the computer is interpreting that as missing signal pulses from those sensors.

I don't know which engine you have or how the fuel pump is powered, but to turn the tables on my story, the circuit that powers the fuel pump may be getting turned off due to missing pulses from one of those sensors. I'm much more familiar with the Chrysler system but many GM models are similar. You may want to connect a voltmeter or test light to the fuel pump's power wire so you can see if the voltage is going away before the rough running starts. Hmac300 might know if there's a tap on the oil pressure sending unit that keeps the fuel pump powered up, or if the Engine Computer keeps a relay turned on like Chrysler does when it sees the pulses from those two sensors. There will always be a way to turn the fuel pump off when the engine stops rotating. That is done to prevent the pump from continuing to run if a fuel line is ruptured in a crash, creating a fire hazard. You could have a low oil pressure problem at idle or a defective sending unit that is killing the power to the fuel pump. That's where monitoring that voltage could provide a valuable clue.

Caradiodoc
Sep 21, 2013.