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Old 08-22-2001, 06:14 AM
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2 Problems Over heating and Rough idle when AC is on, HELP!

300E 1993 3.2 W124
Problem 1 Over heating:
Back ground, does not over heat consistently. In hot weather (not always) and when idle (not always and AC does not have to be on) If on highway (getting airflow) no problem. Getting off the highway and at a stop light temp. starts to rise.
What has been done so far: Replaced Aux fans (dealer said they were binding up) Found out, too late that they work just fine. Found build up of road and foliage debris between fans and radiator, vacuumed it out. Replaced the thermostat. Had independant check main fan and he says it kicks on. This problem needs to be fixed but the dealer wants to throw parts at it and my independent can't figure it out. Please help!!
Problem 2 Rough Idle when AC is on:
This too does not happen all the time. If the AC is on when I start the car feel like I the car is running out of gas and dies, unless I turn the AC off and hit the gas (which does not respond right away). Also, if AC is on and car is running fine, then AC gets turned off and car is parked. When I go to turn the car on again (with AC off) same problem sometimes occurs.

What do I do? Are the 2 problems related? Help???

Last edited by Teddymen; 08-22-2001 at 06:30 AM.
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Old 08-22-2001, 11:16 AM
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Join Date: Apr 2001
Location: Dallas/Fort-Worth
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You have the same year and engine as mine, so maybe I can shed some light on this:

Need a bit more info on the overheating problem you describe...are you losing coolant (i.e. boiling over), or are your engine temps higher than normal?

It gets pretty hot in Texas, and I am OK with engine temps getting near 105C according to the gauge. This is in excruciatingly slow stop-and-go traffic. The aux fans will kick in at about 107C, and lower temps down to about 100C. BTW, this is with the A/C going at full blast.

In cooler days and highway driving, temps stay at around 85 - 87C.

Without getting into the expensive cooling components (which will fail on their own right), I would check two other things...the aux fan sensor, and the expansion tank cap.

The aux fan sensor is located on top of the water pump housing. It is the two-prong connector. If you remove the connection while the engine is running, your aux funs should start going at full speed. They are designed to operate when the A/C is on, since the additional radiator used for the A/C will now be expelling heat in front of the engine radiator. About $30 + shipping at FastLane.

The radiator cap on the expansion tank is desinged to hold pressure at 20 psi or 140 (bar), which is what the cap should say. If it's not doing its job, it can't hold pressure, and thus the boiling point is lowered, so it overheats faster. This is about a $10 part.

Coolant leaves a white alkaline residue, so leaks should be fairly easy to spot. Check for those.

Problem #2 is not related to #1. My first guess would be the A/C idle sensor. Not sure where it's located, but it's job is to adjust the engine idle speed when the A/C is switched on. Other guess would be the OVP. The OVP causes other problems, but when I first had problems, it would be stalling when A/C is on.
2009 ML350 (84K) - Family vehicle
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2012 Mustang V6 (60K) - Daughter's car
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Old 08-23-2001, 05:42 PM
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Join Date: Aug 2001
Location: Florida
Posts: 270
you never know but a restricted fuel supply could cause both problems. An excessively lean engine runs hot and the failure of the rpm's to respond immediately to the accelerator being depressed is also a sympton. Clogged fuel filter???
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Old 08-25-2001, 09:54 PM
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To answer the first question,
I checked all hoses and areas where anti-freeze could be and did not find any residue.
Second question, yes, engine temps are running extremely hot. Before any repairs were done the temps got so hot that when I shut off the car to go into a store then tried to restart it, it did not want to start, but when it did, I had a rough idle and bad response to acceleration. When the fans were changed out and the radiator was cleaned out temps never returned to that extreme. Then the thermostat was replaced and the problem became more sporadic.
I searched the "Good Mechanic" section of the forum and found a couple of independents in my area (Minnesota). I will continue to check all suggestions to my ability then bring the recommendation to the independent I select.
So your continued help is greatly appreciated.
I am due for a Fuel filter replacement and when I bring it to the independent of choice, I will have them do that. My understanding is that there are two fuel filters????
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