Coincidental sensor failure?
Thread Starter
Track Day
Joined: Feb 2022
Posts: 20
Likes: 4
From: Nebraska
Coincidental sensor failure?
I have a crank no start after using an intake cleaner spray. Everything seems to go fine with the spray but the car wouldn't start afterwards. Started with pulling plugs so it could dry out. Then replaced the plugs with new ones. Still nothing. Went through the clarks garage troubleshooting. I have fuel pressure (2.5-3 bar with relay jumped) and spark (tested using an inline light). Tested the crankshaft and speed sensors and was surprised to find both at overlimit. Confirmed at the sensor and the DME plug.
So I placed an order at 944online for new sensors.
Did the intake valve cleaning process somehow effect the sensors? Or was it just a random event?
So I placed an order at 944online for new sensors.
Did the intake valve cleaning process somehow effect the sensors? Or was it just a random event?
Advanced
Joined: Oct 2006
Posts: 97
Likes: 27
From: Tacoma, WA
The cleaning shouldn't have any effect on the speed or reference sensors - they're sealed, magnetic switches. Do you have a tach jump when cranking? You say the sensors tested over limit? The impedance too high? Is it possible you disturbed the sensor wiring during/after cleaning? 40 year old wiring is prone to weird failures
Advanced
Joined: Oct 2006
Posts: 97
Likes: 27
From: Tacoma, WA
I'm not sure what over limit means. >1M Ohm? That'd be a correct reading between pins 25-78 on the reference sensor, and pins 8-23 on the speed sensor. Good reading on pins 25-26 on the speed sensor and pins 8-27 on the reference sensor would be between 600-1600 ohms. Clark's Garage. Your car reads > 1 M ohm on both sets of pins?
That's not a great tach bounce but it does appear to be working. I've see worse where a car ran.
That's not a great tach bounce but it does appear to be working. I've see worse where a car ran.
Thread Starter
Track Day
Joined: Feb 2022
Posts: 20
Likes: 4
From: Nebraska
I'm not sure what over limit means. >1M Ohm? That'd be a correct reading between pins 25-78 on the reference sensor, and pins 8-23 on the speed sensor. Good reading on pins 25-26 on the speed sensor and pins 8-27 on the reference sensor would be between 600-1600 ohms. Clark's Garage. Your car reads > 1 M ohm on both sets of pins?
That's not a great tach bounce but it does appear to be working. I've see worse where a car ran.
That's not a great tach bounce but it does appear to be working. I've see worse where a car ran.
so lets just say its not the sensors.
could the intake cleaner have fouled all four injectors? That seems unlikely too.
im pretty confounded on this one.
Three Wheelin'
Joined: Sep 2021
Posts: 1,251
Likes: 314
From: Phoenix area
If your spark plugs fire, and your injectors fire, then your DME is satisfied with the signals from the speed and reference sensors, without ANY doubt. Look elsewhere for your your issues.
Three Wheelin'
Joined: Sep 2017
Posts: 1,774
Likes: 1,139
From: Formerly the DPRK, now seeking political asylum in Oregon
Longshot...
Did you wash the rings out?
Didn't think it was possible, but I've seen it twice from flooding conditions. A couple of squirts of oil down the plug holes, crank it over a bunch without the plugs to spread it around.
They smoked like a train for a couple of minutes, but were fine forever afterwards.
If the throttle opens at all, the air might be assumed present. If you have fuel and spark, and the spark happens at the right time....
The only thing missing might be compression.
Did you wash the rings out?
Didn't think it was possible, but I've seen it twice from flooding conditions. A couple of squirts of oil down the plug holes, crank it over a bunch without the plugs to spread it around.
They smoked like a train for a couple of minutes, but were fine forever afterwards.
If the throttle opens at all, the air might be assumed present. If you have fuel and spark, and the spark happens at the right time....
The only thing missing might be compression.
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Thread Starter
Track Day
Joined: Feb 2022
Posts: 20
Likes: 4
From: Nebraska
Thats a great idea. I was actually listening to a podcast radio show this morning and a caller had a similar situation. They reccomended pretty much what you said. Ill try it this weekend and report back.
would you use the standard motor oil or some the Marv mystery product?
with everything apart I may go ahead and do a leak down and compression too.
would you use the standard motor oil or some the Marv mystery product?
with everything apart I may go ahead and do a leak down and compression too.
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Three Wheelin'
Joined: Jun 2010
Posts: 1,840
Likes: 58
From: Northern California, '86 951
Did you make sure your injectors are pulsing? Fuel pressure is one thing, but you need the injectors to ground properly.
Last edited by bonus12; Apr 27, 2026 at 04:05 PM.
Three Wheelin'
Joined: Sep 2017
Posts: 1,774
Likes: 1,139
From: Formerly the DPRK, now seeking political asylum in Oregon
General rule in auto repair:
After you've performed work, and a new issue arises...
You ask yourself "What did I just cause?"
Coincidences do happen... But they are bloody unlikely!
After you've performed work, and a new issue arises...
You ask yourself "What did I just cause?"
Coincidences do happen... But they are bloody unlikely!
