1996 850 GLT Random surging/loping at idle and while driv

ERO1 · Post by **ERO1** » 29 Jan 2015, 14:54

Hi All,
Been awhile since I have posted but....I can not figure out what is going on with my car. I have read every post on here that I could find dealing with surging or fuel issues. The car will run beautifully for days ...sometimes weeks and then will start surging when I am going down the road...speed doesn't seem to make any real difference. It will also run like a dream at idle and then start surging ...it will go from 900 rpm to 1500, back down to maybe 500 back up...etc.. I think you get the picture. It seems to be getting worse though. Also when it is doing this it will sometimes blow black smoke...
Ok, so here is what I have done so far...changed the plugs, they were pretty sooty. Checked for vacuum leaks, can't find any, changed fuel filter, checked pressure at shraeder valve....pressure while it was acting up started at 36 psi ,next check 28 psi,then 30, then 26...don't know how accurate using a cheap tire gauge. Also I have had a CEL for awhile.....my friend put in a new timing belt and tensioner...I left from western Va. and got to first rest stop in W. Va. when it came on....don't know if this is relevant...but...I was at Advance a while later and ask them to put their scanner on it ..they said it was missing on 2 cylinders ....So.....what do you guys think....any and all help sincerely appreciated!!!

Thanks, Ruth

Post by **erikv11** » 29 Jan 2015, 15:44

This sounds like a bad MAF or less likely, a bad TPS (throttle position sensor).

One way to test the MAF is run the car for a while with the MAF unplugged. It will run rich (poor fuel economy) and throw a CEL which will need to be reset (Autozone can do that), but it should not surge. If that solves it, get a new MAF. Bosch only, it must be Bosch. Has to be.

A difficulty is the intermittent nature of the problem. So another alternative is to just buy a used MAF cheap, like from a junkyard, and pop that in. But it must be Bosch. If the problem never comes back then you know it was the MAF, if it does come back then you know it isn't the MAF.

ERO1 · Post by **ERO1** » 29 Jan 2015, 16:12

Thanks for the info...in fact I did disconnect the MAF and it made no difference but maybe didn't run it long enough..??? Also...checked vacuum line from FPR and no gas in it...

Post by **erikv11** » 29 Jan 2015, 16:45

Made no difference as in it still surged? Or as in it seemed to run fine? If the latter, try disconnecting it for a couple days.

ERO1 · Post by **ERO1** » 29 Jan 2015, 17:20

It still surged....

ERO1 · Post by **ERO1** » 29 Jan 2015, 17:27

I didn't drive it around because unplugging it did not help the surging problem...

ERO1 · Post by **ERO1** » 29 Jan 2015, 18:35

Another thing...the car starts perfectly, first try. It will die if I let it sit and idle for a bit while the surge thing is going on and don't give it any gas other than that.....
I am trying to think of any thing that might help or be a clue.....I am going nuts...got to have my car running right...kinda afraid to keep driving it like this. The problem does seem to be more frequent.
Thanks..

Post by **erikv11** » 29 Jan 2015, 21:08

OK I see, then it's not the MAF, gotcha.

So it could be fuel cutting in and out, or a vacuum leak somewhere (I know you checked, just trying to make a checklist), or ...

For vac leaks: Did you check the little elbow on the passenger end of the intake manifold, tucked in below the power steering pump? It's a trouble spot.

Tried spraying starting fluid around, to look for vac leaks?

If this car has a fuel pressure regulator (FPR) on the fuel rail over on the passenger side (it probably does), then I would pull the vacuum line to it, and see if there is a smell of fuel in the line. If there is just a hint of a smell that is normal, but if there is a strong fuel smell then the FPR is shot.

The TPS is still a possibility but it usually causes more of a stumble, not a surge. I'd check some other things first.

ERO1 · Post by **ERO1** » 30 Jan 2015, 07:43

Thanks again....when you say fuel cutting in and out,would that be a faulty fuel pump?
I did check the FPR previously by pulling the vacuum line and no fuel or dampness there...didn't smell it though. I wonder about the pressure readings at the shraeder valve that I listed in my first post...should they be that erratic ?
Did spray ether to check for vacuum leaks . Didn't find any. Tried checking the little elbow...seems ok. Also, would a vacuum leak make it act up and then smooth out and run fine? I would think it would do it all the time..?
I have read on here about the coolant sensor too, but my temp gauge seems fine, doesn't fluctuate.

Going down the highway, it will act like it is losing power/not getting enough gas and then it will rev back up...might do it a few times and then be fine again. However, it is definitely becoming more frequent...this has been going on for awhile. Another weird thing is it always seems worse after I fill up...then after it gets down below half the problems almost go away.

Post by **erikv11** » 30 Jan 2015, 08:16

Fuel cutting in and out could be caused by various things, yes fuel pump could be one, or fuel pump relay, or maybe even fuel injector relay. I would try jumpering the fuel pump relay (or just put in another relay, a used one can be had for cheap), drive around, does that fix the problem?

On the FPR the smell has to be strong, faint smell is expected, sounds like you are OK there but you could check again just to be sure. I have never fooled around much with measuring fuel pressure, your observations don't sound too weird, but I will leave that to others to comment on. I do wonder if the check valve on the fuel pump could be failing, giving it intermittent fuel supply.

If you are pretty sure about that last observation then I would look at the top of the fuel tank. Pull the cover over the fuel pump and then go fill up the tank, eyeball it right there at the pump: is there fuel on top of the tank or pump (look at where the wiring goes into the pump, it can leak there), either visibly or by scent? Also check that the fuel lines are securely attached to the fuel pump.