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Acceleration Problem: Update #2

BSD

NAXJA Forum User
Location
OKC
O.K., I posted about a week ago about an acceleration problem I was having that I noticed about the same time I changed out my CPS. Here is a more specific description as well as how I have attempted to remedy it.
The 89 4.0 AW4 seemed to have problems accelerating under moderate to heavy pressure on the foot feed. With minimal pressure on the pedal, it will get up to speed. It will cruise at higway speeds o.k. However, on an incline or heavier acceleration, it will start to miss. There are a couple of variations that are interesting. First, the problem is much more noticeable in when it is cold. Second, it seems to be especially noticeable about 1500-2100 r.p.m. In fact, there are points were it will pick up very aggresively; kind of like the power-band on a 2 cycle. Third, I seem to notice it more in 3rd and 4th. Lastly, if i back off the foot feed, it seems that the missing stops fairly quicly; within about 5-10 seconds.
To remedy, I checked the TPS. It was out of adjustment .173 volts; not much but I made sure it was right on the 82%. The reading on the ground as well as the change while moving the throttle were all good. I changed the CPS thinking that I might have had a faulty one. No difference. The injectors are new with only about 10,000 miles on them.
So, any suggestions? Could this be some kind of timing advance issue? Any feedback would be good.
 
It sounds like it is fuel starved. Is the air filter clean. Check the fuel pressure. If the TPS reads fine and has a nice smooth ramp to the readings, I might think MAP or O2.
 
change your plugs and wires and cap and rotor

first thought in my mind
 
turn off the lights run it after dark and squirt the plug wires with a light mist of water then have someone snap the throttle you will soon see what the problem is dude
 
majic_tech said:
turn off the lights run it after dark and squirt the plug wires with a light mist of water then have someone snap the throttle you will soon see what the problem is dude

huh!?!? Care to elaborate a bit more on that?
 
99XJSPORT06 said:
huh!?!? Care to elaborate a bit more on that?


Magic_Tech is working on the premise that the problem is electrical related...more specifically, an intermittent grounding spark plug lead, coil, or coil wire. Its interesting way to see if you have grounding issues on the secondary side of the ignition system. Trouble is, you are limited to doing this at night. If you've never done it, the key is to find a nice dark area, away from streetlights and start up the engine. Open the hood and have a look inside....make sure the headlights are off as well as any interior lights. Any shortcomings with the spark plug wires, caps, and plugs will usually show themselves as a blue electrical arc. It costs nothing to do and can often point you in the right direction on a weird ignition miss.

I'd agree in that the symptoms could be caused by this ( I was actually thinking about a bad ignition coil) if were it not for the fact that when you back off of the accel pedal, the problem goes away AFTER 5 to 10 seconds. My thinking is, if its ignition related, that as soon as your engine is no longer under heavy load, the problem would stop immediately.

I can't remember if said you've done a basic tune up first, try that first, including the fuel filter and air filter...you probably need those items anyway if its been awhile. I think I agree with Old Man in that this one sounds like it might be more fuel related. MAP would be my first guess.

Dave
 
Thanks all. I will take a look at the filters, check the pressure, do the squirt method. Two questions. First, how do I check the MAP sensor? Second, in checking the fuel pressure. I know it should be 31-35 ith key on, how does one check it under load? It runs fine if you rev it in park.
BSD
 
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