87 Diplomat surging

[Elwood P Dowd]

So I picked up this diplomat a few weeks ago and I've been trying to get her all straightened out but there is still one issue I'm having that I can't pin down. Rolling down the road maintaining a constant speed I get this light surging feeling. The car is so quiet that I can't really hear any perceptible rpm change but I can feel it. Not a lot just subtle. I can't find any vacuum leaks but I have a hunch it's ignition related. It still has the factory lean burn on it and the vacuum line running to the computer has a check valve in it that flutters when I give it throttle. I don't remember it doing that until recently. Any ideas on what to look at next? I just replaced the cap, rotor, plugs, and wires on it.

 

[Darth-Car]

I have never seen a device, check valve, or vacuum amplifier in the line going to the computer. Very odd. Plus it has an open port on it.

 

[Aspen500]

It's been a looooong time since I had a Lean Burn or ESC but that valve doesn't look like it belongs. I THINK the hose should go straight to the ECM. Possibly could cause a surge if its messing with the vacuum signal, which is what the ECM uses to advance ignition timing.
Normally a surge isn't ignition related, unless the timing is jumping all over.
One thing you can do as a test, unplug the EGR hose and go for a drive. I have seen, in the past, EGR valves that the spring breaks or gets weak and then it'll open too far when the solenoid applies vacuum. It creates the same effect as a lean condition. Normally a vacuum leak would affect idle and not at cruise (part throttle, low load), unless it's a BIG leak.
One other thought (yeah I know, look out!). Do you have a lock up converter? I remember my '78 Cordoba would lock/unlock/lock/unlock at 35-40 mph cruise and it was annoying as hell. Felt similar to a surge and at light throttle, the rpm change wasn't that obvious to the ear (no tach). Come to think of it, my '96 Dakota does the same thing and it's annoying on that also.

 

[Elwood P Dowd]

I have never seen a device, check valve, or vacuum amplifier in the line going to the computer. Very odd. Plus it has an open port on it.

That's what I thought too but there is no vacuum leak so I'm not sure what the extra port is for.

 

[Elwood P Dowd]

It's been a looooong time since I had a Lean Burn or ESC but that valve doesn't look like it belongs. I THINK the hose should go straight to the ECM. Possibly could cause a surge if its messing with the vacuum signal, which is what the ECM uses to advance ignition timing.
Normally a surge isn't ignition related, unless the timing is jumping all over.
One thing you can do as a test, unplug the EGR hose and go for a drive. I have seen, in the past, EGR valves that the spring breaks or gets weak and then it'll open too far when the solenoid applies vacuum. It creates the same effect as a lean condition. Normally a vacuum leak would affect idle and not at cruise (part throttle, low load), unless it's a BIG leak.
One other thought (yeah I know, look out!). Do you have a lock up converter? I remember my '78 Cordoba would lock/unlock/lock/unlock at 35-40 mph cruise and it was annoying as hell. Felt similar to a surge and at light throttle, the rpm change wasn't that obvious to the ear (no tach). Come to think of it, my '96 Dakota does the same thing and it's annoying on that also.

I'll take that check valve or whatever it is out and see what happens I'll also take a look at that egr valve and good call on the lockup converter that could very well be my issue. I want to get her running good before I get a catless Y pipe so I can see what kind of a difference that makes

 

[Camtron]

I’d pull the carb and clean it up and rebuild it, if it hasn’t been done yet/recently anyway. Is there an in-line fuel filter that’s perhaps clogged up? Sounds like something is choking your fuel and making you go lean when you’re cruising.

 

[Elwood P Dowd]

I’d pull the carb and clean it up and rebuild it, if it hasn’t been done yet/recently anyway. Is there an in-line fuel filter that’s perhaps clogged up? Sounds like something is choking your fuel and making you go lean when you’re cruising.

I already rebuilt the carb and all things considered it was pretty clean inside.

 

[Darth-Car]

When you sort this out, you will like the catless Y pipe, if you can find one. It is not a big performance enhancer, but you can feel that bit of extra power.

 

[Elwood P Dowd]

When you sort this out, you will like the catless Y pipe, if you can find one. It is not a big performance enhancer, but you can feel that bit of extra power.

I believe summit racing still carries them

 

[Elwood P Dowd]

So I'm finally back home and able to look at this thing again. That little device in my vacuum line appears to be some sort of a vacuum"isolator". If you pull vacuum from the engine side is feels like it's sucking a diaphragm back which would "split" the vacuum between the two ports on the leanburn side. If you try to pull a vacuum on the white side of this thing it just sucks air from the other open port next to it. What I can't figure though it's why it changed how the engine ran when I unplugged the port right at the computer since it doesn't seem like it was getting any signal at all.

 

[Elwood P Dowd]

Just a quick spin around the block seems to indicate this fixed my issue but I'm going for a longer drive tonight to see if it really helped or not

 

[Elwood P Dowd]

Well I do believe I fixed my issue. On a side note she seems to have more giddy up ad go now. I guess that's what happens when the timing is correct lmao

 

[Aspen500]

You took the check valve out, correct? Just curious exactly what you found wrong and good deal it's running smooth

 

[Elwood P Dowd]

You took the check valve out, correct? Just curious exactly what you found wrong and good deal it's running smooth

I'm not sure I would call that thing a check valve. It wasn't open there seems to be a diaphragm in the middle separating the engine side from the ECU side. I'm not really sure how the ECU was getting any signal because it was basically open to the atmosphere.

 

[BudW]

I do remember seeing a valve like the one pictured (post 1) but not one on a M-body before. The computer needs direct vacuum for ignition timing to work correctly. I would bypass that valve completely.
BudW

 

[Elwood P Dowd]

I do remember seeing a valve like the one pictured (post 1) but not one on a M-body before. The computer needs direct vacuum for ignition timing to work correctly. I would bypass that valve completely.
BudW

That's what I ended up doing, I don't know how it was getting any signal at all the way it was setup through that valve