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85 Deville 4100 Driveability

2072 Views 10 Replies 5 Participants Last post by  the APEMAN
This is going to sound like a pretty technical question, but here goes:
I have an 85 Coupe Deville with the 4.1 motor. Car starts perfectly every time and idles just fine. Here's my problem: when the car is warm, I have a "dead-spot" in the acceleration when I push the pedal somewhere between 1/4 and 1/2 of the way down. Once I get past the 1/2 way point, I get the increase in power that I'd expect as I push it down to the floor. The dead spot doesn't exist when the car is drives just fine.

Here's what I've checked/replaced: new 02 sensor, switched the MAP sensors from a good car that I drive everyday, no difference. Fuel pressure is a constant 12psi (well in spec for the TBI motor). Car also has a completely new distributor, cap, rotor, coil, wires, and plugs (all Delco).

Here's what I've observed: my fuel injector pulse width parameter strangely won't increase when I push the pedal down between 1/4 and 1/2 way. It stays at 3.5 milliseconds that entire time (actually it seems to jump up to 3.8, then go down to 3.5 if I push the pedal down). Then, once I pass the 1/2 way point, it jumps up to 4.1 milliseconds, and keeps increasing past that point. Basically, the engine isn't getting enough fuel.

I've checked the throttle position sensor, and its voltage increases smoothly and linearly, and it DOES increase as I push the pedal through the dead zone. The MAP reading (the opposite of vacuum) also increases throughout this dead zone, so the engine is getting the right signals. My coolant temperature sensor readings seem reasonable, as do those for the MAT sensor. Not much else left. 02 sensor toggles between rich/lean voltage. I'm stumped. Misadjusted TPS, perhaps? I can't really adjust it to a higher voltage setting without creating idle coast-down problems (idle would become too low). If I adjust it to a lower voltage at idle, the car becomes more sluggish.

Any ideas?
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My '87 has the same problem. Maybe not quite as pronounced - I'd call it more of a mushy spot or hesitation than a flat spot. A retired Caddy tech told me to put an injector kit in it. The Delco kit (only one worth using in my opinion) is $199 - so for now I'm living with it. seems like a slight miscalibration--or something done to save fuel. Too bad the PROMs aren't easily reprogrammed on these cars. Perhaps similar to the secondary barrels opening on a 4-barrel carb.

I'm going to try playing around with the TPS a bit more since this seems to have a profound affect on the problem.

At least I'm getting great mileage with this car. Never suspected achieving nearly 27 mpg on the highway.
it does sound like your feeling a lack of fuel. The MAP sensor may show an increase, but check it with a vacuum gage. You should have manifold vacuum at idle and when you snap the throttle open sharply it should peg 0"hg for a split second. If the needle is sluggish you have a clogged TB port.
Thanks! I'll check the TB port on the other side of the vacuum hose. I adjusted the TPS slightly forward (my method is that I disconnect the ISC with the motor off, check parameter P.0.1, then adjust the TPS either up or down a degree based on parameter P.0.1). This time I adjusted it up what I felt was about 1/2 a degree. It definitely seems to have helped.
An incorrectly functioning EGR circuit could cause similar slugishness. Also a problem with the transmission might be the cause.

See if disconnecting (just as a test) the EGR vacuum hose (or maybe the EGR solenoid connection) makes a difference.

Thanks! I thought the same thing and tried difference. Now that I moved the TPS up, it seems to have helped. I also sprayed TB cleaner in the MAP port and through the MAP hose. I think the problem has nearly resolved itself.

Has anyone out there driven both a 1985-1987 C or H body car with the 4100 and one with the 3.8L engine (i.e. Olds 88, 98 or Buick LeSabre or Park Avenue)? Is it just me or does the 3.8 seem much more powerful, especially when you put the pedal to the floor? I've had both, and even though the torque ratings are the same, sure seems like the 3.8 had more oomph. I had an 85 Olds 98 with the 3.8 that had 125hp and190 ft-lb of torque (just like my 85 Deville), but it seemed to really move--much moreso than the Deville.

One other question: When it's really hot outside (above 85 degrees), I think that I get some vapor lock with my car. If I shut it off when the motor is warm, and the ambient temperature is hot, my car will stall after about 2 seconds of running when I first restart it. This only occurs if I let it sit for about 10-15 minutes.

It always starts right up at first, but then dies unless I give the gas pedal a quick stomp before the 2 seconds. Once it dies it'll restart just fine.

Any way to prevent this? Not really a big deal, just curious. Car is perfect unless it's above 85 degrees outside. I checked the EVAP purge solenoid and it's working correctly. I think the fuel in the line actually vaporizes since the car starts just fine.

Anyone else have similar issues?
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Are you sure you're not driving my car? Mine does the quick stall on startup once in awhile too.

I think the reason the 3800 seems more powerful is the difference in the final drive ratio. Where the Cadillac is supposed to be silky smooth, the 3800 winds into the power curve much quicker. I remember driving an SSEI a few years back, with the Supercharged Series 2 - OMG :eek:. That thing was quick!
Oh yeah, it is...I was just talking about the regular 3.8 though, even before the "even-firing" 3800 (the 3800 was introduced in 1988 I believe), C&H bodies had an uneven-firing 3.8 in 85-87 (remember, the engine is a 90 degree V-6, not a 60 degree).

Sounds like the hot stall might be a common issue.
Vapor lock used to occur on old carburated engines using mechanical fuel pumps with no fuel return line.

Your engine has fuel injected throttle body and electric fuel pump where the pressure regulator is in the throttle body and it dumps the unused fuel back into the fuel tank. I don't think that vapor lock is possible.

An injector that leaks into a hot engine that's shut off does the same thing.
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