Yeah, somewhere you may have an intake air leak, split or disconnected vacuum hose, or a leak in the throttle body intake area (those bolts can work loose, re-torque: the casting is aluminum). Air leaks seam to show up worse at idle. Other problem could be in the fuel supply line (clogged or kinked/crushed fuel line), clogged fuel filter, gummed injectors or defective fuel pressure regulator or fuel pump(s), there are two on EFI engines. Did you get the fuel line pressure @ 40 PSI at the schrader valve ?. Your fuel tank is also pressurised, is your gas cap good ? If the fuel supply was restricted I'm thinking you would get a misfire at higher RPM's not so much at idle.

If you can find someone who has a scantool, you could get them to measure fuel trims. This will tell you if the fuel system is trying to supply more fuel to the engine due to the lean sensor reading. It will show this by a positive % fuel trim measurement (say + 10 or +15% maybe more, but not over about 25%) . If long term fuel trim is constantly in the + range at varying throttle openings, this points to fuel restriction. The ECU is sending signals to the injectors to increase injector "pulse " width to compensate for the lean mixture. Normally the sensor causes fuel trim to command + & - so the average reading will be zero (or around .5 volts from the 02 sensor) . This I believe is called stoichiometric response & is how the 02 sensor commands closed loop fuel controlled operation. For this to occur though, the engine must run up to over 160*f , if the engine does not reach this temp it will remain in closed loop & will drink fuel ( sooty black plugs & run rich; not like your problem). If you direct propane gas into the air filter box, the fuel trim should change to - negative, indicating that the commanded fuel ratio is to be cut due to rich measured condition in cylinder combustion area from the induced propane. If the fuel trim does not change then your oxygen sensor could be faulty. If it does change then at least your 02 sensor is reading for correct fuel control & is probably O.K.

One thing to note is that the sensors cannot correct for a defective mechanical condition. i.e if your fuel pump or injectors are clogged, the ECU cannot compensate for this but will try to increase fuel flow to correct the erronous readings. It does not know there is a blockage or a defective pump ,only that incorrect reading is coming from the 02 sensor & an adjusted signal is sent to attempt to correct this condition. If condition is not corrected, code will be set. A good scantool will also be able to test the response of the 02 sensor & also check against manufacturers specs if these are supported by the ECU. I beleive with Aerostar, this test is supported.

Couple of q's,
What is your fuel comsumption like?
You changed your plugs what were the old ones like? (colour)
Is this idle condition only when the engine is hot or also rough at cold start?
What does the engine "feel" like on acceleration (power wise).

Good thread......... eventually the answer will come!

 

my fuel consumption seems to have increased only slightly, if any. My old plugs were tossed before i could see them, so i'm not sure if there were any indicators there. The problem does occur when cold, if this was an older vehicle it almost acts like the choke stops working, runs rough and then when i stomp on the gas and rev it up to 3k or so the "choke" kicks in and the idle will increase for a few seconds, then go back to normal and it will run fine for about a minute or so before starting to do it again. The engine seems to have normal power, i drive both residential and highway miles and it doesn;t seem to have any power loss, unless i take off when the engine is running rough, but it corrects itself and smooths out after a bit. I'm about ready to find a professional on this one, as I only have to weekends time-wise to try to figure it out, and i'm afraid i'm going to damage the van running it much more while the problem is occurring.

 

I can't tell from reading this entire thread, but did you try cleaning out the idle air bypass valve? You can also check to see if it's defective by first turning the throttle plate stop screw to manually set an idle speed, and disconnecting the IAC. If your engine idles properly at that point, then the valve is the problem.

Again, I'm not sure if you've checked the vacuum line going to the fuel pressure regulator for signs of gasoline. If present, you will need to replace the regulator.

 

The fact that there are no apparent driveability problems & fuel consumption has not changed, & only cold idle is affected, go for the Idle air valve as noted above. I dont know if this will allow the codes to clear but it does seem like the IAC could be involved + it's easy to check out.


Good luck, Aeroman.

 

sorry for the confusion - it occurs during idle when the engine is cold and normal running temp.

 

look at my final post on "Aerostar" (91 surging/won't idle) it might be as simple as the MAP sensor (can't hurt to pull the plug on this baby ! )

GL

 

took this in to a guy who was suppossedly a retired ford tech - so now not only has the front o2 sensor been replaced but the other as well - next the egr valve & egr sensor was replaced. he's now talking about pulling the entire intake thinking the gasket could be bad... with all the time in the shop the battery gave out so I had to replace it. What where the other sensors that could cause the lurch/surge, i might as well replace what I can and maybe I'll get lucky. Haven't pulled codes since reset when i replaced the battery but i am assuming they are the same as last time.

ps: if gasket or vacuum lines are letting in extra air - could this be tested by ear by spraying carb/intake cleaner around them and seeing if the rpms change?

Hoping i can get this thing fixed, working out of my little mazda pickup is killing me, even if it's saving me money on fuel.

 

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<HR style="COLOR: #666666" SIZE=1><!-- / icon and title --><!-- message --><TABLE><TBODY><TR><TD vAlign=top width="100%">THX ALL who Responded to this thread , my problem is Now " F I X E D " - - it was the MAP Sensor all along, even tho it was unlikely to my trained mechanic bro, he hooked up OBD1 & codes did not remain or show to any great extent, puzzled as we were-he finally yanked the plug on the MAP Sensor & Viola (i missed this revelation, as i was inside reading & typing) i.e , i got out of his hair with my stupid questions & was here persuing what i thot might lend less stupid questions to the repair, i salvaged a new MAP sensor from the wrecker 5$ canadian ! 108 $ retail @ partsource, unfortunatly i lost patients earlier & put in bran-new TPS & IACS AT 43 $ & 106 $ respectivly--Lesson Learned, i dont think a similiar problem can occur without me 1st reaching for this connector & giving it a quick Yank & if that doesnt do it , i might give something else a Quick Yank !! thx again all!
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pull the codes when you can
comments in quote body below

this IAC testing link info will work for your's, slightly different mounting location
https://www.ford-trucks.com/article/...r_IAC_Fix.html
test info the same
can you connect a wire to the IAC white/lt. blue wire and monitor the voltage inside the cab with a DVM while driving, both normal idle and rough idle

 

IAC and Map - i'll look into to getting those and see what happens.

 

alrighty - replaced the IAC - no change.
(The new IAC is slightly different - plug goes straight in the back but otherwise identical, autozone had two kinds, this one most closely matches except for plug direction)

Where is the map sensor located? going to do it next and hopefully it will make a difference.

 

Hi Nightshifter ,

I'm not sure what year Aerostar you have , but in my 91 the map-Sensor is just to the left of the Break-booster/break-fluid resevoir on the firewall when looking from the front end in, it may be covered in that asbestos/felt stuff.... (looks like a thick black box "20-pack of smokes" but a bit thicker/wider with wire's/Plug comming out)

BUT if you "UN-PLUG'' it , you might notice as i did - that your bug should be iron'd out ! , which is an indication that it needs replacing.

Hope that helps

 

didn't see anything that looked like it in that location. Anyone else know where it might be? Going to pull off hutch cover and look fromt he other direction and see if I can spot it somewhere

 

96 4L has a MAF...located in aluminum housing between air box and black air tube to TB.

unplug elec. cable connector...remove 2 security torx screws...lift element out of housing....soak in shot glass full of TB cleaner for 1/2 hr...examine with magnifying lens...wire/ceramic elements should be crystal clean...no dirt, dust or caked on crud...the wires are fragile...no brushing

http://www.autozone.com/az/cds/en_us...rInfoPages.htm

testing, yours is a hot wire with no auto cleaning cycle
http://www.autozone.com/az/cds/en_us...rInfoPages.htm

 

i'm confused - does having a MAF means there's no MAP? and why did autozone sell me one if there isn't one. joy, can this get any more confusing?