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Danny, Shop foreman
Category: Car
Satisfied Customers: 566
Experience:  Senior Master Subaru, G.M.Cert.,Honda cert., Dodge Cert., Ford Cert., 2 ASE Master cert.15 yrs exp.
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Miss at idle, but not all the time..Mass Air Sensor..contaminated

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My 98 Mark 8 with a 4.6 V* engine has started to radomly miss at idle, but not all the time.
The engine will also hestitate on accelaerattion sometimes. I have just recentely had to replace the Mass Air Sensor (MAS) since it was contaminated.
Do you have a check engine lamp on? If so you should have it scanned. If not the most common problem is that one of the coils is going bad or the rubber boot from the coil to the plug is cracked or tron this is very common on the car. Unfortunately unless you want to remove every one of the coils and look at them you will have to have the car hooked up to the ford scan tool so that the cylinders can be graphed to determine which one is causing the problem. I usually replace all of the plugs and all of the rubber boots to prevent this from happening. Unfortunately Ford does not offer the rubber boots for all of the different cars and you have to replace the coil at about 100.00 each. so this is why it is best to know which cylinder is the cause.
Danny and other Car Specialists are ready to help you
Customer: replied 10 years ago.
Reply to Danny's Post: Hi,
Sorry to take so long to reply.
The plugs and boots were replaced about 1000 miles ago. The boots are now available at auto parts stores for ~$6 each.
I'm still looking for an oportunity to look at the coils.
Do you think that I could have burned a valve when the MAP was bad. The car miss fired greatly (engine running lean) befor I replaced the MAP. The symtoms I had that led me to replace the MAP were discribed in a Ford tech pub. Anyway, when I replaced the MAP everything was OK for about 1000 miles, then the random miss started. I have disconnected the MAP intereconnect (places the engine into open loop mode using the memory map) to check the MAP again, but the car still misses. Any ideas?

If it were burned valve the miss would not be random. A coil would cause this however. I have seen many mass- airflow sensor go bad but not enough to damage an engine. Do you have any codes?
Customer: replied 10 years ago.
Reply to Danny's Post: No Codes now. I did have misfire codes before i replaced the MAS. After replacement, they all went away.

Does the engine now miss at idle? or only while driving?

Was it he map or mass air flow sensor you replaced?

Customer: replied 10 years ago.
Reply to Danny's Post: At idle and driving.
I replaced the Mass Air Flow (MAS) sensor in accordence with TSB 98-23-10. I did check the one I was removing and it was defective (one of the sensors (there are two) was open.
Today, I finally had a chance to get to this miss. I removed both covers to the coils and plugs. I moved stuff around, and looked for arcing, but to no avail.

You probably will not see arcing the coils usually go bad internally and misfire as a result. The other thing is to remove and inspect them. However with what you describe the car needs to be graphed while driving to detect the cylinder that is causing the miss. The ford scan tool the IDS will allow us to read misfire graphics on the engine sort of like an old engine scope would. I see a lot of fords that will not set codes but I can find the cylinder that is causing the problem by using the graph. It may be a good idea to have it graphed to detect the cylinder. We do this a lot for the shops around us and generally charge 70.00 to do so.

Customer: replied 10 years ago.
Reply to Danny's Post: OK, Thanks for your help.
I think that you can close this case.