BAD lean spot.... any suggestions??

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rebel715

rebel715

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Hi Y'all, I'm new here and I am trying to make my old slant 6 run better.
Vehicle: 1984 Dodge D100 Pickup with 225 /6.
Totally stock to include leanburn ECU hanging on air cleaner.
A833 4 speed overdrive manual transmission
3.55:1 open diff rear.

Anyway, my problem is this, a SEVERE lean spot, bucking and missing under load from about 1000-1500 RPM's. I have checked and adjusted the timing, replaced plugs, wires, distributor cap, rotor button, carburetor with a new Holley 1945, and I am still at a loss. I have replaced all vacuum lines, and have found no vacuum leaks. I am registering about 22 inches of mercury vacuum at idle and is smooth with a 750 rpm idle. The truck seems to accelerate better when cold and still "on choke". At this point I am suspecting a clogged catalytic converter since my dash mounted vacuum gauge seems to be showing only about 5 to 10" of vacuum at highway speed. I am getting a new catalytic converter next week and will see if that corrects my problem. I suspect the cat because of vacuum readings, the rattling of it, and the fact that the truck ran MUCH better earlier this week when I was on a bumpy road and I presume the honeycomb rattling around shifted and allowed "better" flow. When this occured the truck had smooth acceleration in my 1000-1500 rpm trouble zone. Yes, the accelerator pump is functioning fine, I have tested that too. Any suggestions??
Thanks!!
 
I had a similar problem with a Volvo 240. The cat was cylindrical with cones to taper to the exhaust. The honeycomb became a ball, that blocked the exhaust when oriented against the flow. I needed to take apart the complete exhaust to get all the chunks out. The car had fuel injection so it compensated well for mixture, it just had a great variance in power. I also looked in the exhaust manifold, it was as if sandblasted clean inside. It seemed the car never returned to full power. There may have been some issues due to abrasive wear.

This all happened in a few days that it took to figure out what was going on. A small chunk of honeycomb in the tailpipe was the true giveaway.
 
If it was mine I would take the cat off clean it out (so you can see threw it) unhook the lean bern and have a happy motor.It will run better and more than likely get better gas mileage....but that's me........Artie
 
godfatherofchry said:
If it was mine I would take the cat off clean it out (so you can see threw it) unhook the lean bern and have a happy motor.It will run better and more than likely get better gas mileage....but that's me........Artie
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I would love to!! I am working on a limited budget, so I am just going a little bit at a time. I do plan on replacing the distributor with one with a vacuum advance and going with a Holley 1920 carburetor at some point. I love the truck and want to make it 'right'.
 
I'm wondering if the vac diaphragm on the lean burn comp is blown...but yeah, if the cat is rattling, it's time to cut it out or replace...it shouldn't look or sound like lava rocks :lol:
 
player1up said:
I'm wondering if the vac diaphragm on the lean burn comp is blown...but yeah, if the cat is rattling, it's time to cut it out or replace...it shouldn't look or sound like lava rocks :lol:
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I checked the vacuum diaphragm with my hand held vacuum gauge/pump. It seems to be fine and holds vacuum. I guess I will just wait until after the new catalytic converter and see what that does. Thanks!!!!
 
If it will rev past the 1500RPM range, it's probably not the cat. When they plug, they will usually idle and operate at low RPM's (unless it's totally plugged) but not at higher RPM's. I would suspect the lean burn computer. Can you get it to hesitate and miss just revving it up in neutral? If so, put a timing light on it and see what the spark looks like when you get it to act up. If the advance jumps around or the spark disappears, it's probably the computer.
 
Thanks for the info Cudamark!! I am thinking you are right on the money. I just got the truck back yesterday with a new catalytic converter, muffler, and exhaust and there is really no change. I wonder if my aftermarket gauges are creating an issue. I installed Sun gauges and had to replace the temperature sensor. With the lean burn computer I am sure the old temperature sensor would have had something to do with the running of the engine as a closed/open loop circuit. Currently the wire for the old sensor is just 'hanging'. I guess I'll figure it out eventually!!
 
This might sound stupid. but are you running fresh tune-up parts? bad plugs wires, warped or cracked cap/rotor will give you tons of hell.
 
hemitheus said:
This might sound stupid. but are you running fresh tune-up parts? bad plugs wires, warped or cracked cap/rotor will give you tons of hell.
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No, all of the parts are new and just fine. I just bought the truck last summer and I am still getting it "right". I have also replaced my timing chain and gears and installed a new harmonic balancer. I am just at a loss for the "lean spot" under load at the bottom end of my rev range. I am still using the lean burn electronic spark control module that is hanging off of the air cleaner, and unsure if there is an issue with that. I suspect my aftermarket gauges are causing me an issue with the ECU running in open loop mode since it will be relying on the stock temperature sensor to provide feedback to the ESC to kick into closed loop which will not retard the spark as bad (to reduce emissions). I have checked the vacuum diaphragm on the ESC and it appears to be fine, and I have a good idle pulling 22 inches of mercury on my vacuum gauge.... steady. This tells me I should be alright and is not indicating any vacuum leaks. I also just removed my EGR and cleaned it and it seems to function fine as well. My carburetor is a Holley 1945 and is new and appears to be working fine. I just want to convert the truck to a system that is less finicky. I am just trying to find a way to get the truck running "right" on a low budget.
 
I have to agree with bad cat' converter and/or faulty lean burn ignition.
If you dont have to pass emmisions tests, gutting the converter is just a matter of physical labor. Switching to basic electronic ignition isn't very expensive if you do it yourself.
 
RedFish said:
I have to agree with bad cat' converter and/or faulty lean burn ignition.
If you dont have to pass emmisions tests, gutting the converter is just a matter of physical labor. Switching to basic electronic ignition isn't very expensive if you do it yourself.
Click to expand...
The catalytic converter was my thought too. I just had a new catalytic converter, muffler, and exhaust installed earlier this week, and still really no change in the lean spot issue. I'm stumped!!
 
It seems like some kind of a vacuum leak, but I cannot find any with a propane torch. Actually I see no change at all using the propane, and no change in engine speed even if I stick the open propane bottle into the mouth of the carburetor while running. I would have expected the rpm's to increase, but they did not. Is the lean burn computer compensating for this?
 
One way to rule out the temp sensor is to ground it to the block and re attach that wire. It doesn't absolutely have to be in hot water to send a resistance signal to the ECM. I'm not sure what your ECM would use that signal for. Does your egr valve have a electric controller mounted on top of it ? I think 84 still used a ported vacuum switch only to take the egr in/out of operation. Either way to disconnect and plug that vacuum line will rule it out. I have had to hold the throttle in the sore spot and pinch vacuum lines with needle nose pliers hunting the culprit.
There are just so many little things that cause these problems. Even a faulty fuel vapor valve, electric or vacuum operated, can change the mixture.
 
Thanks RedFish. The EGR has only a vacuum port and appears to be fine. I recently bypassed it, removed it and made a temporary gasket to insert under the EGR from a beer can. No change resulted. The vacuum diaphragm seems to hold vacuum with a handheld vacuum pump. When connected I can move the EGR and make rpm changes with the engine, so I believe it is good. As far as the OASC, I have it bypassed so I am taking vacuum directly around that. I am not sure if the solenoid is good or not. The temperature sensor is currently reconnected and grounded to the top of the engine block with a clip lead jumper. Where is the fuel vapor valve? Is that to do with the charcoal cannister set up? All I know is it seems like a "lean" issue, since when the truck is ON CHOKE it does not exhibit the bucking symptoms under load until it is warm.
Thanks again!!
 
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