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Prom needed/High speed great, mid to idle speeds terrible.

4K views 36 replies 8 participants last post by  gsim 
#1 ·
I have an '84 Sedan DeVille, 4.1 liter engine. Trouble on hwy 500 miles from home. Drove it home. Replaced bad MAT sensor. Was making car run lean, rough below 70 mph, would not idle. Replaced O2 sensor while I was at it because was original. Got new trouble code 52. So replaced ECM (Engine Control Module) Still runs rough at lower rpm and will not idle and still have code 52. Same exact characteristics as I had while out on the road when it first showed up. I am afraid that the P R O M chip is bad and Cadillac no longer sells that. Anyone know where I can get one? Or does anyone have an idea if 52 is a bad ECM or bad P R O M? The 'Service engine soon' and the 'Service engine now' both light up when I read codes on the climate control panel. I think that is pretty much telling me that the ECM is still bad, which leaves only the P R O M since I have a new ECM in now.

Gerry
:crying2: (just replaced exhaust, cat convertor, tires, shocks last year! Only has 124,000 on the clock.)
 
#29 ·
Hold the OFF and Hi buttons after you are in diagnostics! Once you see 7.0 displayed, press them and hold them until .0.0 displays. I'll say again, both service lights WILL be on while in diagnostics. Its the car's way of confirming the bulbs illuminate. Once you see the zeros, press any other button to exit diagnostics. Then re enter diagnostics to confirm 52 is gone. You will see both engine lights come on, a 18.8 display then a 7.0. and NO 52. Try swapping the injector connectors left to right. Pinch the white connector on the top of the injector and lift up gently. If the sputtering injector switches to the other one, you have a wiring issue, if the same injector sputters, you have a partially plugged injector.
 
#30 ·
Thank you, Carnut. I am going to revisit the code thing again based on what you say. I was not able to do anything with the car yesterday. I did discover that I had left a valve open while using my manifold AC gauges to read fuel pressure, so likely that is how it read low but steady @ 6#. I will re-read that today and will get a really good light so I can watch the fuel spray better on both injectors.

One thing I did notice on the voltage test three days ago was that both did remain steady @ 2 VDC while engine was stumbling. Here is the funny thing tho: Driver's side voltage remained @2 VDC after engine died and Passenger side injector voltage died when engine died even tho the key was still 'RUN'. I thought that was pretty strange and am going to confirm that today as well.

Gerry
 
#31 ·
Carnut,
did exactly as you said and codes did vanish. Now have no codes, but trouble still there. Have a steady 5# - 6# fuel pressure, no variation, but READ WITH AN AIR COND MANIFOLD GAUGE THO. Have steady 2 VDC on both injectors while running, no variation when engine stumbles. Did take reading on resistance of TPS. Goes smoothly from 2 1/2 ohms to 5 ohms and no jittering. Figure that part is ok too.

So the only thing that I can point to so far is that I had a bad MAT sensor, and I still have low fuel pressure that is steady @ 5 or 6#. However, it runs just fine @ 70 MPH and above, so I am doubting a fuel pump issue. Am admitting to being stumped for now.
 
#33 ·
Thank you, carnut. I did not know that. I figured that it would give trouble at all speeds if it was the pump. Remember now, it does stumble down and try to die when it is only sitting and idling. And I did read it with an Air Cond gauge, not a gauge made for reading fuel pressure. I did not want to jump to a conclusion, and especially one that involved pulling the fuel tank. LOL

Gerry
 
#34 ·
Thank you, carnut. I did not know that. I figured that it would give trouble at all speeds if it was the pump. Remember now, it does stumble down and try to die when it is only sitting and idling. And I did read it with an Air Cond gauge, not a gauge made for reading fuel pressure. I did not want to jump to a conclusion, and especially one that involved pulling the fuel tank. LOL

Gerry
 
#35 ·
Thanks, Jayoldschool. Will consider. I did not use the proper gauge and it does die when only idling too. Did not want to pull tank and all of that for nothing. Wanted to weigh in with this thing so to speak, and get opinions about it. Another thing is that it runs great at 70 and up and begins to act up at 60 and below. 60 to 30 at least are aldo cruising speeds so it does not cruise well at all except high speeds.

Gerry
 
#37 ·
Hey, Jayoldschool,
If doing 70 and I stomp it, it will downshift for a bit and pick up speed. have run it to 85 with no trouble at all. It does not hesitate/stumble one bit but remains smooth. 65 and below it stumbles as it runs along. 30-40 it seems worse and it will not idle but must be kept running via gas pedal when stopped. Oh, I do recall that I did have the tank off once to re-install the connecting hose, pump-line. No clamps, and not possible to install them. And I knocked it loose last year trying to siphon some fuel for the mower. LOL. I thought it remarkably easy to slip back on and wondered if it should be tighter. Seemed to be a thickish hose, 7" - 8" long, nearly like foam rubber. I should call Cadillac and see if maybe that part is still available before I remove the tank. Maybe if I do all that work I should not put the old pump back either, but put in a new one from NAPA. The car is worth the risk to me so if no other leads present themselves to follow, I will likely try it.

(the 4100 engine is too small for the weight of the DeVille IMO, and from what I have seen both on the highways here and in junkyards here, it was not too popular. Am guessing for that very reason? It tends to downshift more in mountainous terrain if I am trying to maintain 70 or so and that has always been the case.)
 
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