QuestionQUESTION: Sebring 03, 2.4L, Auto, 85000miles. Engine surges while idling (~800rpm) at stop light, rpm jumps up to about 1200 when it surges. Then when trying to accelerate from stoplight .. engine sputters and jerks around 1500rpm .. rpm dances up and down around 1500 while i continue to give it gas. then suddenly it surges and picks up speed.
Then while driving on the highway at about 65mph check engine light came on. The surging while idling continued
at few more stoplights including the jerking/sputtering when tryig to accelerate.
used to ignition on/off sequence to get codes
which are P0068 and then P0172.
recent history: had code P0128 and mechanic replaced a broken thermostat. The above happened two days after the thermostat repair. Took it back to the mechanic .. he could not find anything to fix. cleared the codes and told me all is well. The car drove well for one day. The same problem (surging, jerking/hesitation) occured again today after about 1/2hr of driving with the same two codes (P0068 and P0172).
Can you please suggest a list of most likely things to check ?
when it surges at the stoplight, the car attempts to jump forward as if it gets a suddent boost of power. I have to press hard on the brakes to keep it from moving.
And when i try to accelerate from the stoplight it stumbles and hesitates as if its not getting power ..then
after a few stumbles it picks up and gets power.
Hope you can help with some suggestions for me to request
that the mechanic checks on.
Thanks !
ANSWER: Hi Saf,
The 0068 says that the intake manifold pressure reading when compared to the throttle position sensor and rpm when you are cruising above 2,000 rpm are discrepent. It could be due to a faulty manifold pressure sensor or a faulty throttle position sensor, BUT, on engines that have a plastic intake manifold it is most likely that there is a significant intake manifold leak such as would occur if the manifold were cracked. The recommended first steps are to inspect the manifold for cracks, check the hose from the manifold to the power brake booster canister for loose fittings at either end or a crack, check the PCV valve and hoses for leaks or for a sticky PCV valve. So that is the first order of business.
Beyond that are the inaccurate sensors or problems with their wires that can be checked out.
The 0172 speaks about a too rich mixture as seen by the upstream oxygen sensor on the exhaust manifold, but I would see if you can clear the 0068 problem first because this one might well be due to it. If it persists, then the sensor may be bad, or it too might be due to a fault MAP sensor (which could be involved in the 0068; thus a possible reason to focus on that particularl sensor).
It would be worthwhile to measure that the resistance of the coolant temp sensor is in the expected range (700-1,000 ohm when the engine is hot) as that is located adjacent to the thermostat housing which was involved in the earlier thermostat replacement procedure.
So those are the approaches I would recommend.
Roland
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---------- FOLLOW-UP ----------
QUESTION: Thanks for your prompt response. I did some checking for leaks
and noticed a crack on a hose. It goes from the engine block
to just after the airfilter directly on the accordion-type fat intake hose, but before the throttle body assembly. I think it may be for crankcase fresh air circulation. It is about 1/2 inch or so diameter rubber hose. Not sure what the purpose is. It is not the PCV hose. (the pcv goes from the engine block to the intake manifold just after the throttle plate right?
Do you know what this hose is ? Could this be the vacuum leak that affects the fuel/air mixture?
AnswerHi Saf,
You've named it correctly, "the fresh air make-up hose" which has the purpose of supplying some air to the pcv system if the engine doesn't produce enough vapors by itself. My thought is that it would not likely be the problem in and of itself. But if the pcv valve were stuck ajar then that might be at least part of it, and of course the hose too so that you put filtered air to the intake manifold rather than unfiltered air. But keep looking for something more significant.
Roland
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