4.3 Engine problem

'95 S10 pickup, 4.3 Z (TBI), 4x4, 5 spd manual, 185k miles. It runs fine when cold but once run around for a few miles it starts to do the following. Idle seems ok all the time but when I try to load the engine it misses and bogs down hard. Now here is the trick, if I baby the thing up to 3k RPM it will let me go wide open throttle without a problem (at least that I can hear or feel) but once at lower RPMs again it bogs and misses. So far I have replaced the ignition coil and checked vacuum, throttle position sensor, and timing. Do you guys have any ideas? I was thinking either something ignition wise but that is a stab in the dark. Could the EGR valve or O2 sensor cause this? Thanks for the help.

'95 S10 pickup, 4.3 Z (TBI), 4x4, 5 spd manual, 185k miles. It runs fine when cold but once run around for a few miles it starts to do the following. Idle seems ok all the time but when I try to load the engine it misses and bogs down hard. Now here is the trick, if I baby the thing up to 3k RPM it will let me go wide open throttle without a problem (at least that I can hear or feel) but once at lower RPMs again it bogs and misses. So far I have replaced the ignition coil and checked vacuum, throttle position sensor, and timing. Do you guys have any ideas? I was thinking either something ignition wise but that is a stab in the dark. Could the EGR valve or O2 sensor cause this? Thanks for the help.

Only proper testing will tell you the problem.

Any check engine lights or codes?

Is fuel pressure staying up to par?

Any engine misfires or cylinder miss?

How many miles on tune up?

Good Ac Delco plugs and secondary ign good?

Thanks for the reply.
1. No check engine lights
2. I have not checked fuel pressure yet
3. Engine misfire seems to be my major problem but only under load as stated above
4. 15k miles on tuneup
5. All parts but the wires were AC delco. Everything seems ok but looks can be deceiving.

Thanks for the reply.
1. No check engine lights
2. I have not checked fuel pressure yet
3. Engine misfire seems to be my major problem but only under load as stated above
4. 15k miles on tuneup
5. All parts but the wires were AC delco. Everything seems ok but looks can be deceiving.

95 with a W engine code fuel pressure specs are 55/61 lbs of pressure.

Put a fuel pressure gauge on it and drive it for 20 -30 minutes and watch all pressure readings and post back pressures.

It's a Z code engine. The manual says it should be around 15 psi for the Z code TBI. Is that correct?

It's a Z code engine. The manual says it should be around 15 psi for the Z code TBI. Is that correct?

The Z engine codes fuel pressure specs is 9-13 lbs of pressure.

If engine miss try to find what cylinder is missing and if just missing under load.
Confirm good compression.
Check for any engine running codes.
Check base timing.
Check for dist shaft side play.

Time for an update. I did a basic tune-up, only been 15k miles since my last one, and the truck is running fine again. This is the second time in a row I have had to do a tuneup after just 15k miles due to a fouled plug (both times I used AC Delco plugs). The plugs did look a bit like it is running lean and I had problems with NOx last emissions, which I know can mean a lean mixture.
Checked fuel pressure and got a strange result. When the key is turned on and pump cycles the gauge slowly gets up to just about 10 psi, taking the whole cycle time to get there, and drops back to zero after the pump turns off but key is still on. I understand from the Haynes manual that it should stay up until the key is turned off. What do you think of this?

Time for an update. I did a basic tune-up, only been 15k miles since my last one, and the truck is running fine again. This is the second time in a row I have had to do a tuneup after just 15k miles due to a fouled plug (both times I used AC Delco plugs). The plugs did look a bit like it is running lean and I had problems with NOx last emissions, which I know can mean a lean mixture.
Checked fuel pressure and got a strange result. When the key is turned on and pump cycles the gauge slowly gets up to just about 10 psi, taking the whole cycle time to get there, and drops back to zero after the pump turns off but key is still on. I understand from the Haynes manual that it should stay up until the key is turned off. What do you think of this?

What cylinder fouling plug?
What color is fouled plug?
Blue /black or oil color or white?
Same cylinder each time?

Good hot spark to that plug?

Can you feel engine miss?
Good compressionon all cylinders?
Is it using oil?

What cylinder fouling plug?
What color is fouled plug?
Blue /black or oil color or white?
Same cylinder each time?

Good hot spark to that plug?

Can you feel engine miss?
Good compressionon all cylinders?
Is it using oil?

1. Multiple cylinders (never was able to find out which one, no codes)
2. Plugs had white deposits with little to no brown deposits.
3. All plugs are getting a good hot spark (new wires, cap, rotor, & ignition coil)
4. Feeling the engine miss is what got me here in the first place but now it doesn't miss with the new tune-up.
5. I have not done a compression test and don't plan to if possible, thats a lot of work :smile:
6. Does not burn oil and gets good vacuum (21 Hg).

I plan reconnect the fuel pressure gauge tonight at the filter and dead-head the supply line to see if the pump is putting out enough pressure. If that's good I will dead-head the return line to see if the there is a leak at the injectors. If all is still good then my guess is the fuel regulator is bad. Does this seem reasonable? The fuel pressure shouldn't drop like it does.

As in post 6 have you checked all of that stuff?
Dis you check dist shaft side play?

If plug fouling you should be able to see which one.

We also need compression test readings.
I know a lot of work but we need proper testing to find the problem.

So I ran some fuel pressure tests tonight and want to find out if they are normal.
1. When the key is turned to "on" the fuel lines pressurize to about 11 psi but drop back to zero once the pump stops cycling.
2. I plugged the supply line just after the filter to see if the fuel pump could maintain pressure when cycled. When the key is turned on the fuel line pressurized to about 15 psi and stayed at this pressure.
3. I plugged the return line to see if the regulator or injectors were leaking by into the throttle body. When the key is turned on the fuel line again pressurized to about 15 psi and stayed at this pressure.
So, here are my questions.
With everything connected as normal should the fuel lines maintain pressure when the pump is cycled and key remains on?
If the answer to the last question is yes and the system maintains pressure when the return line is plugged then is it correct to say the regulator is bad, causing fuel to leak by into the return line? Thanks for the help.

Also, just want to reiterate that the miss is no longer present, I am now just trying to figure out what cause the premature failure of plugs. Also, there was no slop at the distributor shaft.

You may have a little fuel pressure leak down with pump off.

But as long as it holds 9-13 lbs engine running you are good on fuel pressure.

If it is running good now with new plugs drive it and see what it does.
Good Luck

its not just a little leak down, it completely looses pressure once the pump stops cycling

its not just a little leak down, it completely looses pressure once the pump stops cycling

Does it hold 9-13 lbs running?

I can let you know in about 3 minutes, just got to hook the gauge back up

it sits at 12.5 psi when running and doesn't change with throttle

it sits at 12.5 psi when running and doesn't change with throttle

That is good.
Fuel pressure is not your problem.

I started checking a few other things and believe my MAP sensor was reading about .5 to 1 volt lower then it should at idle. Installed a new MAP and I am just going to run it for a while, we will see if it goes through plugs again. Is there any other way to know if I am running lean?