My 5.0 will only run in open loop...

[jgonz727]

I have a 1985 Tbird 5.0 that I've converted to a 5.0 HO. It's running a 1989 mustang harness with Mass Air and a A9P computer. The problem I've had since I put this car together is that it's always running in open loop (if I disconnect the mass air sensor, nothing happens). When the engine starts in the morning, it runs fine for a couple of minutes (although by the smell of it, it runs rich—but this might be the case for everyone since its cold). Then as it get closer to operating temp, it starts to misfire and run like crap. It continues to do that for a few minutes, whether I'm stopped or trying to move down the street. After a while, it runs smooth and drives strong—except it still runs rich. My guess is that the computer stutters and convulses when it's trying to go into closed loop, but after a while it just gives up and stays in open loop. There must be a sensor(s) that's not feeding the correct info to the computer, but I don't know which one(s). I admit that it could be a sensor(s) itself or a wiring problem because the harness was acquired from a junk yard wreck. I'm wondering if I can use a multimeter and probe the various prongs at the eec while the car is running to figure out what's going on, OR is there a shop computer that could do this quickly and easily. I have a couple of mustang only shops near me: Chicane Motorsports in Torrance, CA and GTR High Performance in Rancho Cucamonga, but I've had no experience with either place.
By the way, the computer is not throwing the “Check Engine” light at all, unless I disconnect the Mass Air connector. I have done a KOEO test and it does produce two or three codes, but I haven't done it in a long time and I don't remember what they are right now (it's 12:30 a.m.). But when I did do it, the codes were not helpful to me in finding the answer to this problem. (maybe I'll do it again soon, and post them).
Thanks for any input!

 

[fkurls1]

wow! thats allot of info! The only difference between open loop and closed loop is that in closed loop the eec reads o2's. unplugging your mass air and it continuing to run is normal, the eec has stored parameters to run the car with out it. Try unplugging the MAF when it attempts to reach closed loop, if it stops missing your problem is rite there. Also, check to see if the spout connecter is there for the dizzy. When I was 17 I spent a ton of money throwing parts at my 1st mustang only to find out it was missing the spout. keep us updated!

 

[jrichker]

Post the codes you got and I will post the 5.0 Mustang specific causes and fixes for them. The generic code listings aren't much help.

 

[BlownFiveLiter]

I agree with fkurls1 on saying it should continue to run with the MAF unplugged. It just reverts to the tables in the EEC, but it should trigger a CEL with the MAF unplugged. Don't hot plug it though, or you risk frying your MAF electronics, or the EEC itself.

 

[irish]

For sure koeo and koer tests, cylinder balance test, thorough check of all the critical grounds(ecu, 02 harness, strap from engine to firewall, battery ground) these need to be clean and shiny or you get hard to diagnose troubles.

post up the codes when you are done

Matt

 

[jgonz727]

Thanks fellas for the input. I plan to do the koeo test tomorrow after work; I didn't have time today. It's been a long time since I tried, but I remember not being able to do the koer test for some reason--maybe the koeo codes will give us a clue as to why.
Anyways, the spout is connected for the dizzy. But I've begun to wonder about a couple of things that you guys have pointed out: ground for o2's and hot plugging the MAF. I have checked grounds previously at the eec, firewall and I even added an additional ground from a header bolt to the frame, but I've never looked at the ground for the o2's. I'm not sure where to look...I did buy a brand new o2 harness a while back in attempt to rule out a wiring problem with the o2s. Also, on several occassions I did unplug and then plug the MAF back in while the car was running. I hope I didn't burn it out. How do I check it? The Check Engine light would come during the time it was disconnected, but once I plugged it back in, the CEL would turn off. And I also expected that unplugging the MAF while the car is running would have SOME impact on how the engine would run, and it had absolutely NONE--the engine remained idling normal. Interesting, huh??