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Old 10-21-2010, 00:11   #1 (permalink)
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'93 Aerostar Rough Idle, Loss Of Power

After coming to a stop at a red light the other day my '93 Aerostar with the 3.0 V6 started idling rough. Big loss of power too. Almost like it's not firing on all cylinders. Kinda makes an ominous chugging/light knocking sound too. It started acting up in a similar fashion about 7 months ago and I gave it a tune up (changed spark plugs, distributor cap & rotor, pcv valve, & air filter) and it ran beautifully until now.

I had ran it down to about 1/8th of a tank so I thought I might have sucked some bad/dirty gas into the motor so the first thing I did was replace the fuel filter, bought 8 gallons of quality fuel, and treated the fuel system with Seafoam. No change. Next, I replaced the spark plugs. Still no change.

All 6 of the old spark plugs were white indicating a lean condition. Also noticed that vacuum line coming from PCV valve is not connected to anything, just ends near backside of alternator on front side of motor. According to my Chilton's, it's supposed to connect to a charcoal canister which I can't seem to find. Almost sounds like a vacuum leak (I think I hear a hissing sound too when I stand in front of the van with hood up and motor running), but I can't find any obvious sources. Plugging end of PCV hose with my finger with motor running has no effect. I'm wondering if the problem could be a bad oxygen sensor, throttle position sensor, or PCV? Also worried that it may be something worse, like a spun rod or crank bearing. Any ideas anybody?
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Old 10-21-2010, 07:51   #2 (permalink)
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Re: '93 Aerostar Rough Idle, Loss Of Power

Bearing problems wouldn't cause what you're experiencing. Fix the vacuum leak and try this to see what you come up with. Ford Fuel Injection How To Run a Self-Test
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Old 10-21-2010, 10:33   #3 (permalink)
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Re: '93 Aerostar Rough Idle, Loss Of Power

Thanks Bert, I'll give this a try and report back with the results.
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Old 10-21-2010, 16:26   #4 (permalink)
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Re: '93 Aerostar Rough Idle, Loss Of Power

Okay, found my charcoal canister (hiding under the airbox, lousy Chilton's diagram does not make the location clear) and hooked up PCV hose to it; started engine, still running lousy. Still sounds like a vacuum leak somewhere but for the life of me I can't find it.

Performed the KOEO Self -Diagnostic Test, got codes 111 (KOEO) which means system pass; but in CM mode (Continuous Memory), got codes 172 ("No HEGO switching detected, indicates lean[right]") and 452 ("Insufficient input from VSS").

I think I am on the right track, judging by the color of the spark plugs it does seem to be running really lean. But the $60,000 question is why? Anyone care to interpret these codes for me? (Can you help me out again Bert? As an ASE Master Technician you sure bring a wealth of knowledge to the table.)

Thanks again.
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Old 10-21-2010, 18:41   #5 (permalink)
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Re: '93 Aerostar Rough Idle, Loss Of Power

It's very likely a vacuum leak. Check all of the hoses to see if there are any loose. The MAP sensor needs a good vacuum source as do all of the other sensors. See what the engine vacuum is idling.
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Old 10-21-2010, 20:58   #6 (permalink)
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Re: '93 Aerostar Rough Idle, Loss Of Power

Already spent hours scouring the engine looking for air leaks, guess I'll check some more. Engine compartment is really cramped making access really difficult; I'll remove the interior engine cover if necessary. I think I'm going to try a trick I use when trying to find fuel system air leaks in motorcycles and spray some ignition fluid around the fuel injection unit and vacuum hoses; idle speed should change as the fluid is sucked into the vacuum. Hopefully this will help me to find the leak if there is one.

Also checked PCV valve; seems to be working fine.

What does the MAP sensor do? What do you mean by " See what the engine vacuum is idling"?

Thanks once again.
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Old 10-22-2010, 17:21   #7 (permalink)
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Re: '93 Aerostar Rough Idle, Loss Of Power

I know what you mean by the cramped engine compartment. You do need to pull the inside cover off. The MAP sensor lets the fuel injection know what the throttle is doing by checking the barameter and eninge vacuum. The engine vacuum should be around 20" at idle. Spraying as you mentioned is a good way to find out. Be careful of fires.
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Old 10-23-2010, 00:00   #8 (permalink)
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Re: '93 Aerostar Rough Idle, Loss Of Power

Great news! I fixed the van today. So simple, and I never would have guessed it was something so minor.

So after further research I decided to check the MAF (Mass Airflow) sensor as a dirty or faulty MAF sensor can throw error codes and cause symptoms like what I was experiencing:Ford Mass Air Flow Sensors

So I disconnect the MAF, start the engine, no change, still running crappy. So I guess that rules out the MAF. Next, I spray ignition fluid on vacuum lines while engine is running and check for vacuum leaks from the hood. Can't detect nothing. So I turn it off and remove the interior engine cover and search for vacuum leaks. Everything looks fine. So I start it again and continue searching for vacuum leaks on the back of the motor thru the open interior engine compartment cover. A blue spark catches my eye. One of my spark wires is laying on the exhaust manifold and is arcing into the iron. Impossible to see from the front of the motor. I didn't think it would fix the problem, but I knew I needed to move that wire. So I shut it off, move the wire off the exhaust manifold, start it up again, and it runs perfectly! You could have knocked me over with a feather. Couldn't believe it. Cleared the error codes, went for a good 15 minute test drive, it ran flawlessly. Checked the codes again after I got back and the 172 code (No HEGO switching detected, indicates lean [right]) was no longer there. Awesome! Only trouble code present was 452 (Insufficient input from VSS), which could explain why my overdrive doesn't work (VSS=Vehicle Speed Sensor). The overdrive hasn't worked since shortly after I bought the van, (speedometer doesn't work either), but I just drive slow on the highway. Next, I'm going to check the VSS. From what I've read in the Haynes manual, it should be fairly easy to replace. That would be sweet if fixing the overdrive turns out to be simple as well.

I feel like I dodged a bullet here, was thinking it was going to be a major repair (even worried I had spun a rod or crank bearing), so lucky it was just an arcing plug wire. Guess the misfiring cylinder really threw the engine electronics out of whack, hence the 172 error code and lean spark plugs. I had a feeling it wasn't firing on all cylinders.

Took me 4 days to figure it out, but I sure learned a lot about how to troubleshoot computerized engine systems, which is all new to me.

Thanks for all the great help Bert.
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Old 10-23-2010, 06:10   #9 (permalink)
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Re: '93 Aerostar Rough Idle, Loss Of Power

Experience is definitely the best teacher and these type of things is what you will always remember. Good job!
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Old 10-23-2010, 08:46   #10 (permalink)
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Re: '93 Aerostar Rough Idle, Loss Of Power

That's great to hear. Keep us informed on the VSS.
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