1985 Eldorado Engine Stumble & Surge

[carnut]

Check engine to chassis grounds. Should be one behind the alternator on a stud on the engine, one at the rear center hood bumper at the cowl and of course the battery ground. Loose, dirty grounds could be intermittant.

[steelybill]

Carnut is right, bad grounds can cause things you wouldn't believe. The factory manuals show all the grounding points, and there are a bunch, even one behind the corner of the back seat on some models. Mine has one near the battery, which was bad when I bought the car. It has multiple wires grounded there (all waiting to give ya a bad day)

[Elwood]

Again, I appreciate the continued support. Symptoms continue to get worse as the temperatures clime. I now notice a definite miss at idle now that it is “hot” (into the 90’s) and experimenting with gear selection can toggle the symptoms and can get the engine to miss at almost any speed with the throttle just off idle and very low engine RPM. The miss at idle is nearly un-noticeable unless you are obsessed with the problem. I got some time again this weekend (6/6/09) to do some more in depth troubleshooting and found the miss is certainly an intermittent, but possibly simultaneous, loss of spark to the #4 & #8 cylinders? (Note: the “entire” ignition system is new). I inspected, removed, inspected, cleaned, inspected, etc. everything from the distributor to the new spark plugs and all I found was some deterioration of the three connector ECM plug on the ignition control module in the distributor, which I cleaned, rebuilt and covered in heat shrink, to no avail. Measuring with a tachometer at ambient temperature of 70 and engine at full temperature there is no miss until RPM reaches ~1100 and the miss seams to disappear at ~1600 RPM, only missing on #4 & #8 cylinders????? I am all ears!

[steelybill]

The manual for mine also mentions intake manifold gasket leaks as something to check. In the old days we checked for manifold vacuum leaks by using a squirt type oil can with some gasoline in it, and dribbled some along the manifold gasket edges. If it was leaking, the RPM picked up.
Did you get a look at the MAP sensor hose ?, and the MAP hose fitting/opening in the throttle body?
My manual also mentions the HEI ground connection. I'm not sure where thats located on your car. There is a black wire on the coil in the distributor, which is the coil ground wire. Maybe that is what's referenced. The distributor schematic shows the grounds to be on/in the distributor housing itself. The black/red wire in the plug for the distributor is the reference ground to the ECM. Check that connector for loose fit on the pins etc. Mine was problem.
On mine, the ECM , the climate control, and blower motor ground is on the Alternator mounting bracket. You could check if yours is there, and it's condition/torque on the fastener.
Also, there is a junction block on the R/H shroud behind the panel (dash), sort of ahead of the door post area, with 26 connectors plugged into it.
Another one is located behind the panel, to the right of the steering column.

Some of my problems were fixed by simply un-plugging and re-connecting stuff, which likely helped make better contact in the connector pins etc.
That's how I made the cruise control problem go away.

The manual ground schematic shows splices in the ground wires (scary) They are usually in the wire bundles though, and well protected.

Hope all this helps a bit.......
BILL

Oh yeah, the ground at the jump-start junction block near the battery is also the ground for the fuel data center, and climate control panel (control head) That's worth a look too.

[cadillac_al]

I think it is next to impossible for the distributer to misfire only 2 cylinders. All 8 have equal access to the rotor. Now, since 8 and 4 are next to each other, there may be something to it. Maybe there is a defect in the cap, or the rotor may be wobbling pretty bad. I have never heard of this happening before though. I wonder what the plug gaps are on those cylinders. Sometimes I reduce the spark plug gap to .045 when the manual recommends .060 or .080 because I don't believe the HEI can fire .060 unless everything is perfect. When I changed the plugs in my deville they were all gapped at .030; must have been an old timer that put those in. I can't remember if I gapped my new ones at .045 or .060 but sometimes you can't change them that much without putting the electrode at an odd angle.

[Elwood]

Can't thank you guys enough for sticking with me as this thing drives me nuts. I did go through all the grounds I could find, including the one in the distributor, which in mine is one of the 4 ECM harness wires going into the base of the distributor and screws right to the base. Didn't start under the dash yet but I have trouble connecting the dots between an interior ground connection and engine temperature. I've tried the vacuum leak test with carb cleaner and starting fluid to no avail. I commute in this car every day so it's usually too hot for me to pull spark plugs before I'm done for the night, but will run that down this weekend. I've checked all the vacuum connections a couple times but will re-run those again too. I "captured" the miss with an iductive timing light and really could not catch it on anything but the #4 & #8 under any conditions, and the cap, rotor and ignition module are new, and the engine really should only have 87,000 miles on it now and the distributor shaft is still tight.
I'm now thinking that the converter lock is beginning to engage again on coast down which seemed to go away when I replaced the TPS a month ago. The "symptoms" are definately temperature related as it was fairly cool this morning and car ran PERFECT for over 15 minutes before starting to miss/goofy fuel display/etc. and coming home this evening at 78 degrees outside the erratic behavior started to show up in <5 minutes. I'll give it another serious going over if my time isn't consumed this weekend. Still no hard codes set but occasionally get the #23 soft error. I've been wrenching on this "easy stuff" for ~40 years now and I've never seen one do this either.

[carnut]

I think you need to verify the TPS and Idle control motor are at spec. Upon hot restart the ISC should extend then retract after starting to prevent hot restart dying. The Tps should be zero at minimum idle, ISC fully retracted by the book, and base idle at 500 or below.

[Elwood]

Finally got some time Sunday to investigate further. Rechecked all the electrical and vacuum systems under the hood and pulled the spark plugs out of the right bank and re-gapped #4 and #8 to 0.050” from 0.060”. All the plugs (Bosch Platinum ++ ~1500 miles ago) look new. Started and warmed everything up and still have the low RPM miss on #4 & #8 only. Sprayed a water mist on the distributor cap looking for cracks, spark leak, etc. found none. Confirmed the TPS and ISC were within spec also.
When I bought this car is came with a BIG box of parts that the previous owner purchased prior to his “retirement” to an assisted living center and loss of his driving privilege. Most of the parts I’ve installed so far were among these parts in “brand new” in the box condition and I knew there was an ECM in the box so I bit the bullet and dropped the bottom of the dash and pulled my ECM (per the book). I installed what turned out to be a “used” Cardone ECM after transferring the EPROM and started the car. It was still pretty warm from the previous test. The good news is that after 10 minutes of run time on the replaced ECM the loss of spark was undetectable on all cylinders. The bad news is that upon start the engine was loping erratically and when I cycled the throttle the “SERVICE NOW” light came on and the engine idle speed locked at 1400-1500 RPM. The interrogating for error codes indicated a code “100” with all mode lights on the climate control lighted. I shut it down after 10 minutes with no change in this condition and swapped the “old” ECM back into the car. The previous symptoms returned and persist as I commute this week. Both my ECM’s are the cheap Cardone models and do not have the GM part number on them. I’m inclined to go the wrecking yard and finding anther ECM just to see how it behaves but this car can use two different part numbers. Does anyone know how to determine which would be correct? The options are 1226028 or 1226930. My VIN is 1G6EL5788FE647265.
As always, any help is certainly appreciated.

[steelybill]

I don't know about the part numbers, but the ECM uses the pulses from the distributor module to cycle the injectors. You may have to bite the bullet and get an overhauled GM ECM instead of an aftermarket type. I've replaced two this spring in Chevys, from Advance Auto Parts, and they work normally.

[Elwood]

Thanks Bill. I figured out a place to cross reference the Cardone number on the two ECM’s I have and called a couple of my favorite salvage yards only to hit another puzzle. My ECM’s are both part #77-6462, which crosses to a GM #1226462. According to my trusty salvage yard guy and his Hollander’s Interchange this number is correct for the 85’ Fleetwood, not the Eldo. The only used GM ECM I could find was a 1226028, which Hollander’s says is one of the two possible part numbers for my car. However, after your comment that the ECM translates distributor input to injector output I again have trouble connecting a fuel problem to loss of spark to two sequential cylinders, only when hot, and only in a narrow RPM band. Particularly since the spark plugs still look like they only have 1000 miles on them (which they do) and everything else has about 1000 miles on it too. Is there anything else common to the #4 & #8 cylinders that I haven’t thought of?

[cadillac_al]

Maybe those two injectors are clogging and those two cylinders are misfiring from lack of fuel.

[Elwood]

I think I'm on the same track Al, but the HT 4100 runs a TBI so I'm wondering if #4 & #8 are adjacent to where the EGR dumps into the manifold or the air pump may have some affect on the fuel mix making it harder to generate a spark in those cylinders????? I've already closed the gap on those two plugs 0.010", maybe I should go another 0.010"??????

[steelybill]

This may be a dumb idea, but how about switching the connectors on the injectors in the throttle body ?? If the problem remains the same, on #4 and #8, it seems unlikely to be related to the injector output from the ECM. Just an idea

Gapping the plugs closer would maybe give a clue for spark power, but seems hard to figure with all the new parts you have in there.

The manual I have suggests that the EGR port in the manifold can be a problem, and may require cleaning, and that the EGR vacuum port on the TBI for the solenoid can also be a problem. I know you cleaned the EGR valve, but thought I would mention that anyway.

still thinking................

[Elwood]

Thanks Bill. That's worth a try. I'll see if the connectors let me cross to opposite injectors or if I need to build jumpers. I was also thinking I might pull the plug wires off of #4 & #8 and hook them to grounded old spark plugs to see if they lose spark out in the air at the right temp and rpm, just for another data point.
This one realy has me baffled so far.

[carnut]

The injector harness should be long enough to cross over. How about disconnecting the source hose to the EGR and plugging it. Then drive it? If the stumble lessens or quits then we have an EGR thats weak, i.e. lifting early causing a stumble.