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Old 09-02-2019, 02:47 PM   #1
jonah111
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Default 940 hesitation when accelerating

Hey everyone, you'd think that every possible issue out there for redblocks can be found by searching, but I've been doing lots of reading and diagnosis and this one still has me stumped.



1994 940 NA, Bosch ignition (not Regina), 200k- has developed a bad hesitation when accelerating at any speed. This is my sister's car and she claimed that at the same time the problem started while on the highway, she heard ametallic scraping noise for a couple seconds, kind of like when your brake rotor rubs on the sheet metal backing plate. Pressing the accelerator at any speed makes the car bog down and the RPM's bounce around on the tach. The it slowly gets up to speed and is ok once cruising.


Replaced the crank sensor, distributor cap & rotor. No obd codes. Swapped fuel regulator with known good part. Throttle switch is clicking and appears to be working. No vacuum leaks found. In-tank fuel pump is priming when key turned.



Any other ideas on what to check? One think I noticed when swapping the fuel pressure regulators between the 940 and her 240 is that the fuel system on the 240 was under a lot more pressure than the 940. Gas shot out and soaked my shirt on the 240- much less pressure when the regulator was removed from the 940.



thanks!
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Old 09-03-2019, 05:16 PM   #2
dl242gt
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I would go by her described symptom when it happened. Jack up the car and check that each of the wheels spins freely. It may be something like parts fell apart inside the drum brake and are dragging inside where you can't see it. Or a seized driveshaft bearing can cause a lot of drag.
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Old 09-03-2019, 08:42 PM   #3
lummert
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Bouncing tach on my 88 765 turned out to be the ignition power stage (ignition module). But just for kicks check the spade connectors at the ignition coil. I had one of these loosen which I believe caused my power stage to fail due to misfire. I now carry 2 spare power stages.
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Old 09-04-2019, 09:16 AM   #4
jonah111
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Thanks guys, I'll check these items tonight
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Old 09-08-2019, 06:01 PM   #5
Lazarus
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Ignoring the "scraping" noise for the moment, it isn't unusual for the coil/amplifier unit to fail in the older units. Power loss on acceleration would make sense. Also old spark wires that are leaking to the block and of course the plugs themselves need changing. I'll go for ignition troubles on this one.

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Old 09-15-2019, 06:46 PM   #6
jonah111
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Quote:
Originally Posted by Lazarus View Post
Ignoring the "scraping" noise for the moment, it isn't unusual for the coil/amplifier unit to fail in the older units. Power loss on acceleration would make sense. Also old spark wires that are leaking to the block and of course the plugs themselves need changing. I'll go for ignition troubles on this one.

-Lazarus

Are you guys referring to the Bosch LH 2.4 ignition control module (computer) or the ignition amplifier mounted to the inside of the fender (which I'm also assuming is known as the power stage)? I replaced the ignition amplifier, but no change. No stuck calipers or driveshaft issues- the hesitation is also present when in park. Plug wires are Bougicord with less than 20k, spark plugs have 5k.
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Old 09-16-2019, 10:05 PM   #7
Turbo'd 24erty4er
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Perhaps the main fuel pump is not up to spec, the pressure regulator itself or possibly even an issue with the in tank hose from the pre pump, you never know. Best thing to do is check rest and line pressures if possible. Also assuming the fuel filter has been changed somewhat recently. Good idea to also check the injector spray pattern and flow.


Possibility of a weak spark - might be coil related. Obviously something not readily apparent.If there is a tach does it drop suddenly? That tends to indicate an ignition issue. It could even be related to the EZK unit. A bad knock sensor could also be to blame.


Does it ping slightly or run a bit uneven at speed?



One would think if there was some sort of mechanical issue it would manifest itself to some degree possibly one of the pads is dragging on the inner side of a rotor but one would more than likely smell the pad material burning at the very least. Calipers can partially stick.

Last edited by Turbo'd 24erty4er; 09-16-2019 at 10:10 PM..
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