Low throttle hesitation

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Carjunky via CarKB.com

I just put a new (junkyard) engine in my 92 SL2. At idle and above 1/2
throttle it runs like it should. But at about 1/3 to 1/2 throttle the car
just doesn't want to go, kind of chuggs or hesitates. Any ideas would be
greatly appreciated. Not worth shelling out wads of cash to the dealer to
diagnose. It has new plugs, and i checked the tps and it is o.k. Thank
you
 
Hi - just noticed your question and thought I might TRY to help.

I'm thinking in two completely different approaches here:

First, how is the fuel pump? Do you know if there were or ever have
been any problems? Do Check the fuel filter. Are you using the old
fuel injection system from the original engine?

Second - check the resistance for every sensor unit. The computer
might be making the car run "out of whack". O2 sensor, MAP sensor
(does the shift light come on as it should? when it should?) and
Throttle Position Sensor (TPS reports?). You probably want the
documentation to reference these - got a manual and good digital
volt/ohmeter?

My experience with SC2's behaving badly have typically been the engine
coolant temperature sensor which is independant of and located just
below the "display engine temperature" sensor that runs the temp needle
in the dash.

-DC
 
wavy said:
Hi - just noticed your question and thought I might TRY to help.

I'm thinking in two completely different approaches here:

First, how is the fuel pump? Do you know if there were or ever have
been any problems? Do Check the fuel filter. Are you using the old
fuel injection system from the original engine?

Second - check the resistance for every sensor unit. The computer
might be making the car run "out of whack". O2 sensor, MAP sensor
(does the shift light come on as it should? when it should?) and
Throttle Position Sensor (TPS reports?). You probably want the
documentation to reference these - got a manual and good digital
volt/ohmeter?

My experience with SC2's behaving badly have typically been the engine
coolant temperature sensor which is independant of and located just
below the "display engine temperature" sensor that runs the temp needle
in the dash.

-DC
Click to expand...
well to answer your fuel questions, the fuel pump and computer is as old as
the car and the fuel rail and injectors did not come with the engine, i
swapped off the engine i removed. I just finished replacing (took off the
old engine) the MAP sensor, coil pack, and EGR valve. the problem still
persists. As far as the CTS, is that just on the left side of the throttle
body into the block behind the EGR valve? And the TPS i checked to be O.
K. for voltage (.6V at idle up to i think 4.6V wide open). But why would
the CTS cause a hesitation? Do you believe the o2 sensor should be replaced
next?

I have parts from the old engine i can easily swap so if you could give me
more suggestions i would greatly appreciate that. Thank you for your help!

Kevin
 
Oh. and the shift light comes on at redline, but not during normal driving.
 
hello sounds like it is getting to much air at throtle body. check your
vacume harness on top of throtle body and vacume lines for leaks. dirty
throtle body will cause some hesitation.
 
THATs RIGHT... I've forgotten about the basics like
vacuum leaks... Too much focus on the hard stuff maybe?

Oh, but the shift light SHOULDNT come on at REDLINE but
should come on when the rpm gets high enough to shift into
a lower gear (and while taking into consideration of how much
intake manifold pressure exists). Otherwise maybe the Map
sensor is off-measure - maybe due to a vacuum leak...
 
Well, hesitation might be caused by an over-riching the mixture - which
the engine computer easily has the ability to do "if it feels like it".
If the mixture is off, you might experience un-successful combustion
(i.e. engine misses or stumbles).
It might even weaken the sparks effectiveness or strength. (Too much
fuel functions as insulation I heard somewhere ?)

You might also want to see if you can differentiate between initial
"open mode" operation and closed loop. Typically your engine doesnt
become lapdog to your engine control unit until its all warmed up. I
think this can usually be noticed by a drop in the idle rpm. As
reported by the r/f from my spark wires to my fm radio, in open mode -
ignition voltage is at "maximum".
IF it is staying in "open loop", theres somthing wrong somewhere!

(any engine codes? 'check engine?)

coolant and intake air temperature sensors should measure

at 60 deg. f - 3.9k to 4.5k ohms
at 120 deg. f - 1.0k to 1.1k ohms
at 160 deg. f - 430 to 480 ohms
at 200 deg. f - 215 to 255 ohms
(location under front flange edge of EGR unit)
You'll probably need to make nifty insulated pin grabbers and use a
mechanics mirror to guide them on.

crankshaft sensor 700 to 900 ohms

AND - 1996 & Later use Two O2 sensors, the second located just
downstream of cat converter.
they generate no voltage until reaching 600 degrees.
Then the signal varies between
0.1 volts at high oxygen and lean mixture burn to
0.9 volts at low oxygen and rich mixture burn.

-DC
 
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