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I have an 1989 mgm with a 5.0 engine.The engine would fluctuate a little, which i debt with it. It got worse. I replace the throttle body with both sensors(throttle &amp;iacv). Also replace the EGR gasket &amp; upper manifold gasket. Change most of the vacuum hose even the one in the manifold. Change the PVC valve and disconnected the battery to reset the whole thing. Still have the same problem. The only way it stop fluctuating is when i disconnect the vacuum line that goes into a round plastic container. When I disconnect it, it run smooth but if i would connect it back. It would fluctuate again. Does anyone knows why.Thank you for your answers.[img]smileys/smiley14.gif[/img]

 

have you checked the oxygen sensors? they might be caked with carbon and not functioning quite right. opening the vacuum line is probably making the engine race a little (increase the rpm as if you were holding the accelerator down just a bit) and that will always smooth out an engine. my 88 mgm fluctuates a little at idle, but runs great. I may need to check my oxygen sensors too, but as long as it passes inspection, I won''t worry about it. As long as your 89 doesn''t stumble (nearly die on you) and continues to run, I wouldn''t worry too much about small fluctuations in idle. if it''s fluctuating at higher RPMs too, then there''s an issue somewhere. have you checked the air injection system (should be on the passenger side going to a pipe that connects to both heads around the back side of the engine) if that thing is leaking it can cause all kinds of vacuum headaches. (I had to replace the crossover pipe cause it was rusted through) Also check the vacuum pressure sensor and the line from the intake plenum (top section of the intake manifold), if the line is cracked, you''ll need to replace it (bugger pain in the *** to do so cause you have to take the intake plenum off to do it) and replace the sensor if it''s bad (though I''m not sure how to test that).

There are probably other sensors and vacuum lines to check out, but this should be plenty to start with (and it''s all I can think of right now)

 

Both oxygen sensors were changed around four years ago, around 20,000 miles. The disconnected vacuum line has my car running smooth. RPM is at 625 in drive, which is therecommentedRPM by ford. I haven''t lose any power at all. Yes, it does pass the emissionbut since i change the oxygen sensor, my emission number have gotten bigger.Maybe i will put back the old oxygen sensor and see how it will react.The cross over bar are in good condition. The vacuum lines at intake plenum were replace last week with the gasket too.Where is the vacuum pressure sensor located at. Also, I don''t get the check engine light on, so i don''t know what to check. Anyway, this weekend if it not cold because i am in the Northeastern, I will check the vacuum diagram and get to work.I will get back to you as soon as i am finish with it, and I wish you all a Merry Christmas &amp; A HappyNew Year.... Thank you verymuch.
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the vacuum line that comes off the back side of the intake plenum goes to the vacuum pressure sensor mounted to the firewall near the heater core (at least on my 88, guessing it''s in the same location). I''m leaning away from that now and thinking there is some restriction in the air filter, intake snorkel, etc. Might check it to make sure you didn''t suck a plastic bag into the air box.

 

That sensor is callMAP sensor on my car.When I disconnect the vacuum hose from that sensor the car starts idling real fast.Any vaccum line disconnected from the intake plenum, makesthe idle too fast. The vacuum line that i disconnect is the one next to the battery that goes to a black round cannister like. It is a red plastic hose that is connected to the black cannister. Maybe, you can see it in your 1988 MGM, MR Slymer and see what happen when you disconnect the hose. Let me know the results.I really appreciate all your information which is helping me solve my problem. Thank you very much SIR.......[img]smileys/smiley14.gif[/img]Edited by: pdman

 

do you mean the small round one on the wheel well or the large one under the battery?

if you mean the small one on the wheel well next to the blower motor, that may not be the problem. it could be the switch driving it.

if you mean the large one under the battery that has 2 hoses going to it, that one could be clogged and need to be replaced. it could also be the valve that''s near the battery on the top side of the wheel well or just above the frame rail that has failed. (at least looking at my 302)

looking in my chiltons (I have the 68-88 and 89-98 books) it looks as if the cars are actually different in their vacuum diagrams, but it could just be that the 89 is easier to look at than the 88.

 

&lt; =text/&gt;_popupControl();</> I mean the smallon to the wheel well, next to the blower motor but if it is the switch driving it. Why does it throw out air. Maybe, you are right I will see if i can test those switches before i decide to spent money. Or even play with the electrical connectionsto see what happens.Anyway, do you know how to test those two vacuumswitches. Edited by: pdman

 

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After, doing some research the name cannister next to the wheel well is call vacuum resevoir and the other ones with the electrical wiring are called solenoid valves. I have replace most of the the plastic vacuum lines with the rubber hoses. After replacing most of the vacuum hoses, the car responded great for around 5 hours and then started to fluctuated again. So, I had to disconnect the vacuum hose that goes to the vacuum resevoir. So,my next step is to to test the egr and see what happens. I will inform these forum of my outcome. Thank you and Merry Christmas to all.

 

good luck

 

&lt; =text/&gt;_popupControl();</> I found out that i had chris-cross two vacuum lines. I read the vacuum diagram that is under the hood and so far the car is responding great. Vacuum lines are very important towards your cars performance. Anyway, so faris responding good and i hope it stands like that for a long time. Well, I will let you know in a few more day if anything positive/negative happens. Thank you and have a great day.[img]smileys/smiley14.gif[/img] Edited by: pdman