'95 Blazer miss/surge under light load

[exmodifiedracer]

My '95 Blazer misses/surges? under light loads when the engine is warm. It runs fine when cold (when it's it the open loop mode). As soon as it warms up enough to go into closed loop mode the miss/surge begins. It idles fine and runs great when accelerating at wide open throttle. Only problem is cruising along under light load. The check engine light has NOT come on.
I've put in new spark plugs, wires, cap, rotor, oxygen sensor, fuel pressure regulator, fuel filter and egr valve.
I've also tried putting in a different ignition coil and ignition module.
None of this has made a difference.
Found a couple of vacuum line leaks that I repaired. This provided only a minor improvement. Checked for leaks around intake manifold and found none.
Looking for any other suggestions before I throw in the towel.

[old_master]

Fuel pressure and ignition are not monitored by OBDI or II, (95 could have either one). You've got both primary and secondary ignition pretty well covered, have you checked fuel pressure?

[534BC]

Your symtoms are identical to mine and it (mine) was the spark leading away from the distributor post when advance over about 32-34 degrees. It happened only under very light load (slight decel to slight accel) and only when in closed loop.

I never did find the actual cause , but think it is the timing chain stretch or distributor gear/bushings worn . I cheated and rotated distributor body ccw to bring the posts closer to the spark when advanced.

[exmodifiedracer]

Originally Posted by old_master

Fuel pressure and ignition are not monitored by OBDI or II, (95 could have either one). You've got both primary and secondary ignition pretty well covered, have you checked fuel pressure?

Yes.
Gauge reads 55/56 @ idle, 59/60 @ 45 mph

[exmodifiedracer]

Originally Posted by 534BC

Your symtoms are identical to mine and it (mine) was the spark leading away from the distributor post when advance over about 32-34 degrees. It happened only under very light load (slight decel to slight accel) and only when in closed loop.

I never did find the actual cause , but think it is the timing chain stretch or distributor gear/bushings worn . I cheated and rotated distributor body ccw to bring the posts closer to the spark when advanced.

So, what you're saying is retarding the timing cured your problem? At this point I'm willing to try most anything so I guess I'll give it a try. Thanks.

[534BC]

No, on mine the timing is controlled by the computer and rotating the dist body doesn't change timing. Think of it as rotating "just the cap only" It changed (lined up) the rotor to cap relationship and was really just treating the symptom. Many miles later I have never found the actual cause and really don't care to ,.,;' yet. I'll look for some links.

[534BC]

here's my original thread and my problem started out very small and took 2 years to get big enough that was detectable. I got off pretty easy without r&ring too many new parts that were'nt needed. A coil and plugs maybe.

http://www.automotiveforums.com/vbul...d.php?t=607221

I can't seem to find the threads talking about the timing and such, but I think that if your distributor has a fork and roll pinned to block (making it non-moveable) and has the posts coming out the sides rather than the top then it is consideredd the "high voltage switch" and is just like mine vi code W 4.3

I cheated and broke the plastic tang/fork off the distributor body so I could swing it like an older one. If my problem ever comes back I will pull distributor out examine gears bushings and drop it back in 1 tooth different since I rotated it up against the intake and it has no more travel.

[exmodifiedracer]

Originally Posted by 534BC

No, on mine the timing is controlled by the computer and rotating the dist body doesn't change timing. Think of it as rotating "just the cap only" It changed (lined up) the rotor to cap relationship and was really just treating the symptom. Many miles later I have never found the actual cause and really don't care to ,.,;' yet. I'll look for some links.

I rotated the distributor approximately 5 degrees CCW today and, lo and behold, it worked!!! Vehicle performs better all around. I don't know why, as you said, but who cares! Thank you!! Thank you!! Thank You!!
Saw your most recent post too. My distributor has the posts upright, not to the side. All I did was loosen the clamp on mine and rotate the distributor slightly. Hope it lasts.

[534BC]

You actually changed the timing then in addition to the sprk relationship. Even though you have the different set-up than mine you and I probably have the same root problem.

[BlazerBoyLT98]

Also check your MAF sensor, give it a good cleaning or replace it if it is bad

[exmodifiedracer]

Originally Posted by BlazerBoyLT98

Also check your MAF sensor, give it a good cleaning or replace it if it is bad

Thanks, but no MAF on my '95.

[BlazerBoyLT98]

Ah good point

[old_master]

Originally Posted by exmodifiedracer

Yes.
Gauge reads 55/56 @ idle, 59/60 @ 45 mph

Here is the procedure for fuel pressure check:

Install the fuel pressure gauge to the test port near the distributor. Turn the key to the RUN position but do not start the engine. While the fuel pump is running, fuel pressure must be 60-66psi. After approximately 2 to 3 seconds, the fuel pump will shut off. Fuel pressure must remain between 55-60psi for several minutes. A drop in pressure below 55psi indicates excessive leak down. Possible places to leak are the fuel pressure regulator, the fuel lines under the upper intake manifold, a fuel injector that is stuck open, a leaking fuel line or connector, or the check valve in the fuel pump assembly.

Accurate diagnosis of an internal fuel leak requires additional test equipment. In some cases it requires the removal of the upper intake manifold.

Check it using this procedure and post your results.

[exmodifiedracer]

Originally Posted by old_master

Here is the procedure for fuel pressure check:

Install the fuel pressure gauge to the test port near the distributor. Turn the key to the RUN position but do not start the engine. While the fuel pump is running, fuel pressure must be 60-66psi. After approximately 2 to 3 seconds, the fuel pump will shut off. Fuel pressure must remain between 55-60psi for several minutes. A drop in pressure below 55psi indicates excessive leak down. Possible places to leak are the fuel pressure regulator, the fuel lines under the upper intake manifold, a fuel injector that is stuck open, a leaking fuel line or connector, or the check valve in the fuel pump assembly.

Accurate diagnosis of an internal fuel leak requires additional test equipment. In some cases it requires the removal of the upper intake manifold.

Check it using this procedure and post your results.

Checked fuel pressure as per your instructions. 60 psi after pump shuts off & it remained at 60 psi ten minutes later.