SL2 stumbles/hesitates on acceleration
 

Hi, I just bought a 2001 Saturn SL2 about a month ago. Almost immediately the code P0410 popped up and it turned out to be a bad air pump. I changed that and had no problems for a couple of weeks. Then I came home from work one day and it started to idle rough and the MIL was blinking. The only code it gave me was random cylinder misfire so I researched a little and replaced the ignition module, coils, wires, and plugs. That fixed the problem but now I have a new problem.


The engine idles perfectly now (maybe a little high) but when I accelerate it hesitates for a split second and then lunges forward. I've scanned for codes and it's not giving me anything! I checked the gap on the plugs and made sure they were at .040, they are. It even died trying to take off from a red light once (embarrassing). I don't know where to go from here, help?

Does the throttle stick? That is when it is at idle, it takes a little extra pressure to get the throttle to start to move, particularly if it has been idling for a minute or two? If so, you need a throttle body cleaning. If you want to do it yourself, let me know and I'll post the instructions.

I dirty throttle body is often caused by an EGR valve that stays open a little too much. The EGR gasses blow right on the backside of the throttle body, gumming it up.

Is the car warming up as it should?

Thanks for the suggestion. I don't think the throttle is sticking. I mean it feels normal anyway. The car does warm up quickly however the gauge never reaches the middle line. I don't know the proper operating temperature but just judging by the gauge on the dash it seems to run a little colder than normal. I did just replace the ECT sensor but that didn't do anything.


Just for more info, I did check the TPS using a voltmeter and it seems ok. Could the idle air control valve cause this condition? I also suspected a clogged fuel filter but it has good power at high RPM's so I don't know. Oh, and it also seems to only happen after the engine has warmed up.


I don't have a lot of experience with Saturn so I could clean the throttle body if you think it might help.

Its not the idle air control, that would not cause this problem. Everything points to either a vacuum leak or low fuel pressure or possibly dirty injectors. Injectors don't usually all clog up at once though, and a vacuum leak should result in a high idle, unless it is a very small leak, and a leak that small should not cause this problem.

On my Saturn, the coolant temp gauge stays about two needle widths above the 1/4 mark. Last summer when I was in Death Valley when it was 121°F, the needle climbed to about one needle width above 1/2, but that the highest I've ever seen it go.

A clogged fuel filter will be noticed when you are WOT at near redline. Then the engine will start to miss or just loose power suddenly. So I agree that is probably not it. My next guess would be the MAP sensor.

But before going there, I would want to put an analog voltmeter on the front O2 sensor to check its response time. There is a condition, just before these finally start setting a code where the sensor is operating within its voltage parameters, but is getting slow to respond. It should sweep between its lower and upper limits about once a second, but when they stat to slow down to once every 3 or 4 seconds, then you can have this issue.

If the sweep is normal, then I would look for either a resistance problem in the MAP sensor wiring or a pinhole vacuum leak very close to the MAP sensor, like maybe its seal or remove the MAP sensor and see if its port is partially clogged. If there is a delay in sensing the change in manifold vacuum, again the computer will be delayed in adjusting the A/F ratio and you will get a stumble.

Thanks for your help, sorry for the late reply. I did check those things to no avail but I finally found the problem. Apparently when I installed the new plug wires I crossed #1 and #4. I just switched them to 4,1,2,3 and it accelerates smoothly now. Go figure

Yep, that would do it. Sometimes you luck out and discover it's something simple. Congratulations.

I thiught that since 1 and 4 are in the same coil it wouldnt matter with post the wire for 1 or 4 is on dont they fire together at same time?

There is a capacitive plate under the #4 side of the coil that is your cam position sensor. Switching these wires is like putting your distributor in 180 degrees out, which would not be a problem for a lost spark ignition, but it causes the fuel injectors to pulse out of sequence.

If you had switched #2&3, you would never notice it.

Thanks yea I'm glad it was something simple. Guess I'll pay better attention next time. I wonder though why the symptom only occurred after the engine warmed up. Could it have something to do with open and closed loop?

You dump a lot of extra gas in when the engine is cold, it would mask the problem.