mass air problem

stang0f87 · Apr 9, 2005

i converted over to mass air some time ago on my 87 gt. its all stock except gt40 heads, xpipe, and flowmasters. anyways it always takes a dump at idle when i first fire it up. after it warms up for a few minutes its fine. if i drive it on and off all day it doesn't seem to do it more than once. after it sets a while(like overnight), it's gotta be warmed up again. takes a few minutes to idle right. i've double checked the wiring and it seems in order. is this normal behavior?

kj2833 · Apr 9, 2005

have you checked out the idle air control motor on the throttle body. One that age is bound to be full of black carbon goo. Just cleaned out my buddys 87 last week, was doing something similar to what you described, even though he is still SD.

dstanggt50 · Apr 9, 2005

I would check codes to see if anything comes up, are you running a stock meter, maybe if you know someone that has another meter, give it a try. I converted to maf in my old 87gt conv, (same longblock in my 95) , it had a horrible idle problem, but it was both when cold and warm. We had to get some idle adjuster (don't know if thats the right name)that goes onto the iac.

stang0f87 · Apr 9, 2005

thanx for the responses. pretty sure its not the iac. i had that problem before too. i'll check it again to be sure. yeah i do have another meter but its for 24 lb injectors. suppose i could throw in my 24s and try that meter as a last resort. as for the air adjuster, i tried that too. bought one off ebay. plan on probly using it on my 347 when its built. no luck with that either. i'm new to mass air. i have heard thats it normal for a mass air car to need a warmup period to adjust to mods. any truth to that?

kj2833 · Apr 9, 2005

at this point im not sure if what you think is happening is actually happening. Put a fuel pressure gauge on it some morning before start and check it out. it could be bleeding pressure off while it sits overnight. I would start checking for vacuum leaks as well as some loose wires, distributor issues as well. all of these can and do act up when engine is cold moving to warm.

either way, some thorough testing is in order.

Daggar · Apr 9, 2005

kj2833 said:
at this point im not sure if what you think is happening is actually happening. Put a fuel pressure gauge on it some morning before start and check it out. it could be bleeding pressure off while it sits overnight.

This part doesn't make much sense. Of course it's bleeding off over night. These things are not as tight as a drum and that part doesn't matter. When the pump primes (if it's working correctly) then the desired pressure is reached just about as fast as you can turn the key to the start position. Have you ever run a Stang out of gas? What heppened when you put gas in it and primed the pump. It turns over a few more times than it normally would then fires right up. This portion of the post doesn't apply to the problem at hand nor does is even indicate a problem. Fuel pressure bleeding down over a period of time with the pump depowered is not a malfunction.

stang0f87 · Apr 9, 2005

that reminds. its got kind of a fast start when i fire it up. don't remember when that started. been that way for a long time. running a 190 lph fuel pump. i'll check the pressure when i fire it up tomorrow morning or later tonight.

kj2833 · Apr 9, 2005

Daggar said:
This part doesn't make much sense. Of course it's bleeding off over night. These things are not as tight as a drum and that part doesn't matter. When the pump primes (if it's working correctly) then the desired pressure is reached just about as fast as you can turn the key to the start position. Have you ever run a Stang out of gas? What heppened when you put gas in it and primed the pump. It turns over a few more times than it normally would then fires right up. This portion of the post doesn't apply to the problem at hand nor does is even indicate a problem. Fuel pressure bleeding down over a period of time with the pump depowered is not a malfunction.

i must be misinformed then, a ford tech told me that they shouldnt bleed down very much at all, and when they do they usually have a bad pressure regulator. he said some bleed down is normal but that sometimes they do it real bad. He also talked about how sometimes they will start right up but the pressure will bounce around and dip below the cutoff, mostly hiding the pump as the culprit. This will also allow you to Discount the pump and start looking at regulator, injectors.

stang0f87 · Apr 9, 2005

any other ideas would greatly be appreciated though. so is it true that mass air cars need to be warmed up?

Daggar · Apr 9, 2005

stang0f87 said:
any other ideas would greatly be appreciated though. so is it true that mass air cars need to be warmed up?

Start going through the suggestions you've gotten so far. I'm not sure how much the sensor itself has to do with how the car runs at startup. TMOSS or jrichker might be more helpful in that particular area. When the car is cold your car is running in open loop. It means that you're running form a preset fuel table that's not taking inputs from many of your sensors. Try first to pull codes and see if that gets you anywhere. It would be a shame to just blindly start throwing parts at it only to find out that your EGR is sticking when it's cold or something silly like that. If you're not getting any codes, then pull the battery cable and let the thing sit without power for 30 minutes or so. The preset fuel tables that I mentioned above are (in part) a product of the computer's ataptive strategy and may have been part of some bad data. If you're able to get your hands on another meter just for test purposes, the try it out. There's got to be someone in your area that can help you out with that one. Let us know how it turns out. PM TMOSS and jrrichker. jr's got a checklist for damned near everything.

Ranchero5.0 · Apr 9, 2005

Also as a quicky go back over the puter connector and make sure the conversion pin outs are still locked in and making contact. Had that happen on a customers car.

Jamie

jrichker · Apr 10, 2005

IAC (aka IAB) isn't working. Clean it, clean its electrical contacts & clean the 10 pin connector contacts.

stang0f87 · Apr 10, 2005

ah hah. i think i found the culprit. i was nosing around the back of the intake and discovered a vaccuum hose that ain't hooked up to nothing. i felt around and i think it leads directly to the back of the intake underneath where u can't see. i'm thinking this is the vacuum used to run to the map sensor thats supposed to get plugged off but i'm not sure. gettin dark to see. can anyone tell me where that vaccum hose runs?

JDubz84 · Apr 10, 2005

I believe one runs the the PCV valve, and the other goes to the MAP sensor. I could be wrong though. Take the vaccume line off of the back of the intake and plug it with a screw. This would explain our problem, mine did the same thing since I forgot to plug the line. Hope this helps.

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