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Old 09-30-2017, 03:53 PM   #1
504
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Default 1986 245 M46 Won't rev past 2000 rpm

Over the past couple days, I had noticed a bit of hesitation while driving, but it seemed to pass quickly. Today, I found that the hesitation is due to the motor refusing to rev past 2000. If I hit the gas hard, it might "bounce" past 2000 momentarily, but then drops right to 2000. (I'm watching this on the tach.) If I ease up, it just stops there. I can still drive, more or less, but my acceleration is slow and shift points are early. If I try to ease it above 2000 it's like something is just cutting the power.

WTF?
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Old 09-30-2017, 04:40 PM   #2
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I'm also stalling a lot at idle.
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Old 09-30-2017, 09:59 PM   #3
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Try unplugging the MAF. Does it change how it runs?
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Old 10-01-2017, 04:06 PM   #4
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Today it ran fine. If it does it again (which I think is likely) I'll unplug the MAF.

Further research also suggests a bad MAF could be the problem. Does the MAF tend to fail sporadically before failing entirely?
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Old 10-05-2017, 08:11 PM   #5
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Did it again for a minute or two.
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Old 10-05-2017, 10:44 PM   #6
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Put a gauge on the fuel rail and watch the fuel pressure while driving. I have had this be a main pressure pump intermittent either from a bad relay, bad wiring to the pump, or the pump itself failing. Or all of the above.. lol.
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Old 10-06-2017, 10:12 PM   #7
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I replaced the fuel pump pump relay relatively recently and keep a spare in the glovebox. I also replaced the fuel filter somewhat recently and checked the wiring at that time. No obvious damage.

Any links regarding an appropriate fuel pressure gauge? How exactly do you hook it up so you can monitor it while driving?

What indicators suggest a fuel problem vs. an intermittently flaky MAF?

So far my problem has been intermittent. Does the main pump ever fail intermittently? Or is it generally a total failure?
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Old 10-06-2017, 10:29 PM   #8
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Could you have kinked the fuel line when you replaced the filter?
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Old 10-06-2017, 10:42 PM   #9
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kink the FPR return line and see if it revs up better..
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Old 10-06-2017, 10:57 PM   #10
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To be clear, it revs fine most of the time. So far the 2k limitation has been very erratic.

Re: possible kink in fuel filter line. I checked the log, it was replaced a little over 2 years ago, so I think that's unlikely.
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Old 10-06-2017, 11:22 PM   #11
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On a 2.2 car, you don't have much to go wrong. How is the engine wiring harness?

MAF could be getting ready to die I suppose.

Same with main pump.
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Old 10-06-2017, 11:28 PM   #12
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I've seen fuel pressure regulators stick closed and cause the pressure to spike.

I was having a tough time diagnosing a customers intermittent poor running condition and when I found nothing wrong, I told him to go ahead and have his car tested for emissions. Apparently, while on the rollers the regulator stuck and he failed the test. He brought the car back to me and of course, the car was running fine. I connected a fuel pressure gauge and let the car idle. When it suddenly started running poorly, the fuel pressure gauge was reading full pump pressure. Changing the regulator cured the problem.
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Old 10-07-2017, 08:40 PM   #13
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Those intermittent issues are the worst. Sometimes you have keep trying at it like that.

In answer to the question about fuel pumps. Yes, I have had main pressure pumps fail intermittently and the only way I caught it was by monitoring the pressure while driving. When the problem would happen I pulled over and checked the system pressure and it was low something like 45psi. It was enough fuel pressure that it would idle fine but there was no power and acceleration was...well there was none. This was a kjet B21F in a 1981 wagon I had a long time ago. It was M46 and had a 3.31 rear making it the slowest 240 I ever had. Looking back that emissions cam in it really hampered how it ran.
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Old 10-07-2017, 10:09 PM   #14
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With the fuel pump issue you had, was there a hard limit at 2,000 rpm or some other spot on the tach?

When it does happen, it has been very consistent in stopping right at 2,000. If I give it more gas, it just makes a weird sound but won't rev any faster.
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Old 10-07-2017, 10:21 PM   #15
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Well it made so little power you couldn't rev out a gear like even first or second. I don't remember any specific rpm though.
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Old 10-08-2017, 12:20 PM   #16
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What you are describing is typical behavior with a failing AMM.
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Old 10-08-2017, 12:45 PM   #17
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I just replaced a bad 2.2 amm/maf. My symptoms were a rubbish idle, then as soon as I would hit the gas it would stall flat on its face. If I held the throttle wide open it would sputter and backfire out the intake. Super odd.

Installed a used maf and it idles great now and revs freely
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Old 10-08-2017, 01:17 PM   #18
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my 89 was doing stuff like that and I unplugged the connector at the MAF and cleaned it with deoxit. it's a good contact cleaner, some of the cheaper ones don't work well. problem was that. didn't need a new MAF. Suggest trying that first. look for green contacts. you might unhook the battery and work the connector in and out like 30 times while it's wet with cleaner to clean the pins. then wipe as best you can with a little screwdriver and thin cloth. blow it out, then dry off and try again.
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Old 10-08-2017, 01:34 PM   #19
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another time I had that symptom after I had rebuilt my engine. I got the plate that the TDC sensor reads out by a bolt hole so it couldn't read TDC, had to pull the tranny to move it to the right spot.
I dont' suspect the plate is wrong but maybe you could have a bad sensor. In your car it would probably be a distributor hall sensor. I have an 88 740 with that. in 89 they went to the TDC sensor. It had problems reading the signal from the hall sensor and was intermittently dying. I ordered the distributor plate with the hall sensor mounted from rock auto. it ran good 6 months after that , more recently my 88 740 stopped making a spark and it's dead. i think it's a bad ignition module or mabe the central computer. I thought maybe the sensor went south again and ordered a whole distributor from ebay, it was a cheap knock off replacement distributor but came with the cap, rotor sensor and everything. still no spark though..

I don't think it will start if it doesn't see a pulse from the hall sensor.. Im thinking MAF is more likely. If it dies intermittently then I'd check the hall sensor, the o rings in the distributor wear and the sensor's beam can get messed up by the oil. If it looses the pulse it'll kill the engine.

with my 1990 with the out of position plate , it ran , wouldn't rev high. I think it was seeing a pulse but it made the timing so far out it couldn't rev up. Yours doesn't have that type of sensor.

Last edited by amazonphil; 10-08-2017 at 01:44 PM..
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Old 10-08-2017, 08:17 PM   #20
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MAF contacts are clean and dry. I have also cleaned the MAF recently with MAF cleaner.

I have posted in WTB.

Interested in getting a spare MAF, perhaps it will also fix my stalling issue.
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