sounds right. try subbing in a new one. before you do that, make sure the vacuum source for the map is pulling correct vacuum and that the hoses/lines are in good condition. that sounds like you have it nailed.

 

You're absolutely right! The voltage should be low with vacuum (at idle) and close to 5V with no vacuum.

My auto instructor told us about the same problem he was having with school's S-10. They had recently bought a MAP sensor that exhibited the same reverse voltage problems. It turned out the sensor they got was for the turbo version (I think).

 

that's freaky weird. This MAP has been the one that's been in it since I bought the vehicle. Guess I'm buying a new one!

 

Hmm...Autozone confirmed that the MAP was a useless piece of plastic...tested the new one and it responded as expected. Engine still misses under load/acceleration though...

 

Have you made sure the port from the TBI is clear ? Even the smallest pc of carbon
will mess up manifold absolute pressure's signal. Since it is an older ride,
did you also get a chance to test the TPS ? Another thing that was brought to
my attention is the dist..Is the advance working as it should ? Is the dist even
in phase ? They get old too. I'm thinking of getting MSD's dist and trying it
out, but I had such a crappy time with one of their ign box's I'm leary..

I don't know what to say about the miss under acel..Since the MAP really
has taken over the job of the vacuum advance in old style dist's it would
point to it being the culprit, but then there's correct timing that is
needed from the EST up on the firewall via the knock sensor..
Hey might be as simple as a stretched timing chain.

I guess the only way to get to it the bottom of it, is to run the ign on a scope and the
MAP, TPS, Timing on a graphing DVOM to watch for clean voltage increase/ decrease
or timing increase/decrease. That's why that dist machine on Two Guys Garage was the sh*ts
yesterday..Man that thing worked great.

 

Hmm...Autozone confirmed that the MAP was a useless piece of plastic...tested the new one and it responded as expected. Engine still misses under load/acceleration though...

check your plugs and wires for carbon tracking. that sounds like your misfire issue. also, change the plugs while your in ther and make sure you are not burning oil. it fouls out plugs on the old 4.3l's all the time. that stems from the valve seals getting rock hard and leaking oil into the cylinders. you might notice blue smoke on start up. very common for the 93 and older 4.3's.

 

lol I've done hte plugs (and wires) twice. The first time I put in Bosch Plats. After making that mistake (and posting it...ha!) I quickly yanked them and put AC Delcos back in. In the mean time, my plug wires got caught in the steering shaft and the insulation was a bit torn, so I decided I'd better just be safe and get another new set. I've also replaced ::inhales deeply:: the cap, rotor, coil, ICM, ECT, Fuel Filter, Computer, and Distributor. Timing chain seems tight (Engine only has 60k on a GM rebuild...)...

I think a 12 ga. slug at high velocity may be the best help

Oh yeah, if I think the cat maybe plugged (I've seafoamed it twice and someone said that may do it) can I run the engine with the exhaust pipe removed or is that really gonna screw stuff up? I heard it could becuase of the cold air on the valves or lack of backpressure or something.

 

Oh yeah, if I think the cat maybe plugged (I've seafoamed it twice and someone said that may do it) can I run the engine with the exhaust pipe removed or is that really gonna screw stuff up? I heard it could becuase of the cold air on the valves or lack of backpressure or something.

If the engine can't breath it can't run. A partially clogged cat has the same effect on
an engine as putting your hand over the bores of the TBI. It will kill it, but before it
does you'll see and hear an actual decrease in power and performance..Clogged
cat is the same thing. Works on the theory of suck, squeeze, bang and fart. Can't
fart, won't go..

 

Fixed it last night....turns out I was getting about 3psi on the fuel line....new Bosch pump and Trogdor roared right to life!

 

Good for you i still havent figured out whats wrong with mine it sounds like its doing the same thing as yours, and i have replace the same stuff as you did and more. i think im just gonna rebuild it and hope it works..

 

runs with a hatchet, (with a hatchet), i'm a juggaluggalow!!!

 

Good for you i still havent figured out whats wrong with mine it sounds like its doing the same thing as yours, and i have replace the same stuff as you did and more. i think im just gonna rebuild it and hope it works..

hey, whats yours doin?

 

If the engine can't breath it can't run. A partially clogged cat has the same effect on
an engine as putting your hand over the bores of the TBI. It will kill it, but before it
does you'll see and hear an actual decrease in power and performance..Clogged
cat is the same thing. Works on the theory of suck, squeeze, bang and fart. Can't
fart, won't go..

actually, it won't straight kill. you will be able to drive it when its ice cold and as the cat heats up, it starts expanding the catalyst and chokes the engine out. it will start to lose power, it may stumble, but it will feel like you have a semi trailer in tow. you can drop the y-pipe when things are hot and drive the truck a short distance to comfirm the cat as your culpret. if you can drive it normally, then the cat is junk. if it still bogs, you have another issue. possibly an egr sticking open. don't drive it too long as you can set fire to things underneath.

 

The best way i can describ it is that it lacks power. when i stomp on the gas its like im just hittin the gas half way. i cant even spin the tires going in reverse then hittin it into gear. that how much power it lacks.. idk what to make of it..

 

you've done plugs, wires, cap, rotor, coil, ICM, pickup, ECT, fuel filter, fuel pump, knock, map, etc? Have you checked TPS, compression and fuel pressure?

 

I dont beleave i have done the knock sensor, but i have done everything else, i replace the throttle body with a holly, i replaced the brake booster(Possible vac leak). i did check compression they where low but withing 15psi from each other.

 

I read you can check the knock sensor by putting a timing light on it then having someone tap the block with a hammer. The timing should jump back each time you do it. I also read you shouldn't run with out the wire connected because I guess you can jack your engine up pretty quick if it is knocking.

How low are the compressions?

 

Rear Pass - 120 PSI
Rear Drive - 130 "
Mid pass-140 "
Mid Drive-135 "
Front Pass-130 "
Front Drive-120 "

Where exactly is the knock sensor located?

 

LoL. I thought it was the thing between the 2 and 4 cylinders on the pass side., but turns out that is the temp. sender for your dash (wtf? Why can't they just use the ECT?), anyway, the knock is ACTUALLY behind the head on the driver side, kinda down past the oil pressure switch. PITA to get to. Mine disconnected by squeezing the entire base (if I remember right) and pulling up. Also, it's a single dk blue wire going to it.

 

Ok i will take a look at that when i have a couple minutes of day light

 

sounds good. You want to make sure your plug wires aren't running parallel to that blue wire either (and preferably away from your ESC unit too) because they can induce a knock signal if they are close enough. If I remember right, disconnecting the Dk Blue makes the computer think there is no knock, even if there is, and disconnecting the ESC unit will make the computer assume there is a knock signal, even if there isn't.

 

So if i disconect the blue wire from the knock sensor if that is what is causeing it, it should straighten out?>

 

In theory, but that's risky because if the engine is really knocking and the knock sensor is delaying the spark legitimately, then you could trash your engine. Have you taken a timing light to it to see where the timing is at? You could have a friend hold the brake and put it in drive to put some load on the engine and see if the spark advances...

 

Yeah i checked the timing a couple of times it seems to be in the right place. i will try the under load thing this weekend maybe