Ideling issue and dies on hot days while driving

[mgm89]

I have a 89 mgm for about 2.5 yrs now and the same issue has praqued it the whole time. I have changed the iac, the tpms, most of the vacuum hoses, smog pump (with used) I got one 02 sensor changed and the other unplugged since I can't get it out. The problem is no matter hot or cold it will start fine and about a min or 2 of idling it will rev up and down and eventually die unless I push down the gas pedal. Also on very hot days it will die when idling or driving after about a 30 min drive and will then start right back up.

[Kodachrome Wolf]

Sounds like symptoms of a bad TFI module. Heat eventually kills them. Sometimes they die outright, other times you’ll get intermittent running followed by the engine cutting out, then being able to restart it after it cools down a little. I experienced a similar situation on my ‘87, except after I limped it back home, it restarted once for a few moments, and never started again until it got a new module installed.

Some auto parts stores can test those modules, but an intermittent issue may not crop up on the test bench. If you request for one, it’ll usually be listed as an ignition module. Folks will usually recommend Motorcraft, but if that’s unavailable, get a good quality one. The cheaper ones are more likely to fail sooner.

Another thing to check is fuel pressure and make sure the fuel pump is providing sufficient pressure at the rail. A pump that’s going out will give trouble too.

[mgm89]

i changed the pump with one of those universal ones but am not sure on the pressure. it makes a shit ton of noise tho.

[mgm89]

i have a bunch of parts i bought to replace and i got one of those tfi modules along with the air filter and thermostat. i guess i just got to change it.

[sly]

EDIT: Forget everything below. Get your O2 sensor fixed. With one unplugged, when it switches to running fuel trim, it requires both sensors to run right. That's what's causing your issue.


If the idle starts ramping up and down, check the vacuum line to the MAP sensor. Should be mounted up on the hood seal lip behind the engine a little passenger of center. If that vacuum line is soft, crumbly, or otherwise crap, it may be getting soft enough to close itself up and give bad readings. If that's not the case and that hose is good with no cracks and seals good to the ports it's on, it's something else.

It could also be a bad coolant temperature sensor or air charge temperature sensor (on the lower intake behind cylinder 5 - 1st one on the driver side). You can test these to make sure by checking resistance and making sure they are actually changing in accordance with the temperatures. Check them cold, plug them in, run the car to get it warmer, then check again. Values are here:
ECT: http://www.grandmarq.net/oldfuelinjection/page28.html
ACT: http://www.grandmarq.net/oldfuelinjection/page29.html
I had a case once where the ECT was stuck on some intermediate value and would never change. Replaced it and that fixed the running issue, but mine didn't die, it just ran badly.

[gadget73]

What they said. Plug the O2 sensors in, confirm the air and coolant sensors are good, and check for vacuum leaks. Also make sure the O2 sensors are connected on the right side. If somehow the harness got flipped around, running the left sensor on the right input and vise-versa makes it really stupid.

On that subject, there is a ground wire coming out of that harness that needs to connect to the back of the engine. Should be orange, and it usually lands on one of the rear-most lower intake bolts. If you have a random wire hanging out back behind the intake, thats probably it. Its the ground reference for the engine sensors. It will throw the values from the O2, ECT, and ACT if its not hooked up.

[knucklehead0202]

EDIT: Forget everything below. Get your O2 sensor fixed. With one unplugged, when it switches to running fuel trim, it requires both sensors to run right. That's what's causing your issue.


If the idle starts ramping up and down, check the vacuum line to the MAP sensor. Should be mounted up on the hood seal lip behind the engine a little passenger of center. If that vacuum line is soft, crumbly, or otherwise crap, it may be getting soft enough to close itself up and give bad readings. If that's not the case and that hose is good with no cracks and seals good to the ports it's on, it's something else.

It could also be a bad coolant temperature sensor or air charge temperature sensor (on the lower intake behind cylinder 5 - 1st one on the driver side). You can test these to make sure by checking resistance and making sure they are actually changing in accordance with the temperatures. Check them cold, plug them in, run the car to get it warmer, then check again. Values are here:
ECT: http://www.grandmarq.net/oldfuelinjection/page28.html
ACT: http://www.grandmarq.net/oldfuelinjection/page29.html
I had a case once where the ECT was stuck on some intermediate value and would never change. Replaced it and that fixed the running issue, but mine didn't die, it just ran badly.

^^^This. I chased weird idle shit on my first '90 and it got better and better, but was never fully fixed until i replaced the IAT and ECT sensors, after everything else of course. Defo get that 02 sensor un-fucked.

[mgm89]

i might end up taking the manifold off the get the 02 sensor off and i know the rear ground wire is connected cause i ended up replacing the connector on it when i tore the top of the engine apart. i thought it was the 02 sensors for a while so i changed the driver's side and the passengers side is the one i am having difficult to get off. i have the special o2 sensor socket and wrench socket thing. the vacuum line to the map was changed and i was wondering is the map could cause that issue. i just removed the coolant temp sensor and put in a gauge it i need to put it back it i kept it anyway. i never checked the air charge temp sensor.

[gadget73]

There are two coolant sensors. The one wire one is for the idiot light, and you can remove that. The two wire one is for the ECM and it needs to be there or the engine is not going to run right.

[mgm89]

the one i took out is on top inside of the intake manifold it had one wire hooked up with it.

[EaOutlaw1969]

If this was my car i would start with basic dignostics, however since you have a O2 sensor disconnected you will have to re connect it and like mentioned earlier make sure the bank connections are not crossed.

I try to always go back to the basics when it comes to diagnosing problems even if the basics do not seem like they could be the cause of your issues.

Since the car has had this problem when you purchased it, why not get to know your car better, you could run a compression test a fuel pressure test, check for stored codes in the computer.

The fuel pressure test kit and other tools needed can be borrowed from most parts stores like Napa, pepboys autozone ETC. yes you have to pay for the tools then get a refund once you return them but in the end you are not out any cash.

As far as diagnostic tools I used to like for older cars like yours was a Snap on Scanner MT2500 with the troubleshooter cartridge and the cartridge for the year and make car your driving.

The Snap on Troubleshooter is loaded with helpful test to make diagnosing your car easy well easier than the shotgun method.

I understand if the Snap on is too costley to buy but you may get lucky and have a friend close by with one that would be willing to help you diagnose it.

If troubleshooting the problem is not possible or running the few test I mentioned i am not sure what to tell you, other than like the others have mentioned the Thick film Module could be the problem yet I would add you should inspect the coil and secondary ignition system for signs of high voltage arcing and check the pick up in the distributor for signs that it is on the way out.

Normally the insulation on a failing pick up will soften up and be easily marked with your finger nail or even crumble apart with the slightest touch, As a rule when i could afford it when i replace the TFM I also replace the pick up module making sure to check for play in the distributor shaft while the distributor is out.

If i remember correctley. you can disconnect the spout connector for the ignition timing and if the car runs better that is a good indication of a problem with the TFM or pickup if there is no change neither part is likley your problem.

As far as the Stuck O2 sensor you will have to deal with the issue somehow even if it means replacing other parts, yet I would try heating the manifold if the sensor is not allready mangled from previous attempts to remove it.

Another thing to look out for is a out of adjustment throttle stop screw, If someone cranked it up to make up for a broken vacuum line or other problem this could cause other problems.

To check this you would need to make sure the Throttle posistion sensor is sending the correct information to the computer, for example if you probe the signal wire and it is reading 2 volts with the key on engine off or running at idle and goes up smoothly to 5 volts at wide open throttle with the key on engine off that would be a good indication of a misadjusted throttle stop screw.

The most helpful thing you can do for yourself is get a good service manual either a online manual or real paper type service manul from Ford.

If you have access to the manual along with help you can get here for all of the helpful members fixing your car up should be pretty easy.

[DerekTheGreat]

Unfortunately, I have a Snap-On MT2500 and all it will let me do is see what codes are being thrown and self test things which don't do much other than tell you what the code does, if the self test even performs. Ford engineers in the 80's were assholes since they wouldn't let you look at live data without the big ole ugly break-out box. That didn't change until the early 90's for most cars. I know with our '92 F150 we can look at live data which is very handy..

[EaOutlaw1969]

If you have the snap on MT 2500 and are disappointed with it, it may be that you have not explored the troubleshooter cartridge.

Back n the day with that tool a dvom and a manual There was not one single computer related problem on any car it would hook to that I couldn’t fix.

That being said you have to be willing to run the test, heck if I remember correctly the MT 2500 even had a power balance test you could run to let you know if you had a cylinder down.

I took my MT 2500 over to my friends house and helped him with his 1990s F 150 he had a no start, he had replaced the Thick film module and a number of other parts.

I showed him how to run the diagnostic test with the troubleshooter cartridge and we were able to pin point his problem down to a sheared distributor gear pin.

If I remember correctly the troubleshooter even suggested techs reporting it to be a common problem with the inline 6.

Sure I would have eventually found it without the troubleshooter or the scanner yet when properly used the MT 2500 can save a lot of wasted time and money, but if you do not use a bucket load of common sense while diagnosing a problem the MT 2500 can lead to disappointment and part swapping shotgun method.


The sad part is I gave my Snap on MT2500 to my friend and told him to keep it as but to never sell it, iI made him promise to give it back if he no longer had a use for it.
I found out that just months later the jerk sold it on eBay.

I had every cartridge known to man for it yet he just sold it like it our nearly 30 year friendship didn’t mean as much as the few hundred bucks me made off my tool.

This video is kind of lame yet it gives you just a taste of the potential of the Snap on MT 2500 in the right hands

https://youtu.be/gUCg9rxiz6Y

[DerekTheGreat]

Guess the first sentence of my last message is all you read haha. I like my MT2500, I don't like 80's Fords for being all proprietary and shit. GM let you stream live data right from the start with their crap and yet Ford made us wait until the early 90's or buy that dumb break out box or back probe stuff in order to see what was going on. Troubleshooter cartridges and stuff are cool. My MT has paid for itself already with diagnosis. Try and hook it up to a box Panther though and you won't be able to see much other than codes and the troubleshooter info that'll go along with it.

Wow, some friend. I can't stand people like that.. If anything they'll give some lame excuse to justify why they did what they did. Funny thing is, before you give them or do anything for them they'll sit and agree with you, promise right to your face and try & tell you how great & respectable they are but then turn around and do stuff like that. I tell ya... I still have my Dad's old Firebird, I'll always have that car. I told him I wanted it when I was 10 and even though there were rough periods I managed to keep the damn thing despite the odds. Then I hear similar stories of a relative passing a cool car down to some ungrateful kid who simply sells it within a year. Makes me cringe. Especially when the car was owned for decades and/or the whole reason it was held onto was to be passed to that family member.. I don't look forward to thinking about where my cars & stuff will go..

[knucklehead0202]

Got a few family firearm heirlooms that will be passed down to my children in similar fashion. People that sell shit like that are dicks. Just bought an MT2500 and some cartridges. Can't wait to try it out.