1986 Eldorado stalls under stress

[FrankC]

Thanks for the guidance hthorn. I believe I have a vacuum gauge, not sure, but if not, I will get one, borrow one, or get mine back and test to see whether the cat is bad. You describe an interesting procedure to make that determination. I plan to better understand why the vacuum reacts the way it does depending on the condition of the cat.

I'll advise of the results I find.

Thanks again!

Frank

[hthorn]

Cool--just remember that if the exhaust has too much backpressure, it stays in the cylinder too long, thus displacing the fuel/air mixture. To develop vacuum (since an engine is, essentially, a vacuum pump) there are many interdependent factors. The fact that your transmission has been shifting late as you describe indicates a faulty vacuum signal, as the vacuum cannot build to a sufficient level to allow the transmission to shift. It's being kept low by the exhaust gas which cannot escape fast enough.

[FrankC]

Thanks again hthorn. I appreciate that explanation. Reminds me of two other factors that may contribute to that discussion.... One is that there has been some black smoke coming from the exhaust pipe for perhaps 100,000 miles. Not hardly enough to notice, but there is more than there should be. (A friend, who was following me one day when the car had perhaps 75,000 on it, commented that some black smoke was coming from exhaust. Car has 177,000 miles on it now). 2nd: Brake pedal goes too low. I have bled the brake lines a lot... like I've bled more fluid at one time than the lines can hold and there is no leaks. Perhaps something else going on there besides low vacuum, but, just thought I would throw that in.

Another issue I have been curious about is the auxillary vacuum pump that starts up when the ignition is turned on. I assume the vacuum from the pump helps the power brake unit and other vacuum dependent accessories. Are the two vacuum systems kept separate somehow? Perhaps there is no connection between the power brakes and the vacuum created by the engine.

I know this is a difficult question, but, if a new cat would have perhaps a 2" diameter passage way for exhaust through it, how small could that capacity become and still allow the engine to run as well as mine does? Say... 1", 1/2"...? I hadn't realized how these symptoms could tie back so readily to a plugged cat. It would be interesting to bypass or gut the cat just for a test.

I have to heal for a few days from two hernia repairs, but am anxious to get back out there to replace ignition components and test for defective cat.

Thanks again for your continued support.

Frank

[Edahall]

The CAT has very small honey comb passages. They can easily become clogged.

If you live away from the city, you could certainly unbolt the cat and drive the car with a pair of ear plugs.

[hthorn]

You can either bypass the CAT or use the gauge--my guess is your neighbors will appreciate it more if you use the gauge! It will tell you definitively. Much simpler. Maybe once before you do the ignition work, and once after. Then after the exhaust work is completed, you should be able to feel the difference. But if you can find your earplugs, and not your vacuum gauge...
Black smoke could mean incomplete combustion, i.e. unburned fuel, which makes it now 99% likely that your CAT is defective.
...Auxilary vacuum pump? Not unless you have a diesel. You might want to check your master cylinder. Low vacuum would cause a "hard pedal". Also, if you're not bleeding the system properly, you might not be getting all the air out.

[FrankC]

Thanks guys. Yeah, I live in a neighborhood that would not appreciate muffler removal. It is hard enough to live here with an '86 Eldorado. I can't find a vacuum gauge in the garage, but Harbor Freight has them for around $13 with the fittings and hose. So I plan to pick one up tomorrow.

hthorn... My car does have an electric (14v @ 8 amps) vacuum pump. It is located under the battery tray. While I never had a car with one before, I just assumed that all the Cadillacs had one. I would think the pump would affect the vacuum test regarding the cat. Indirectly, it is tied to the brake booster, cruise control, egr, transaxle modulator, MAP, etc. Might make one wonder whether that could be a source of transmission problems. The pump is noisey.

The brake pedal isn't hard. It just goes a long way down. And I did follow the bleeding procedure to the point that I really don't think there is still air in the line. I have wondered whether the master cylinder is bad. But, I figured if it was allowing fluid to get passed the seals, the pedal would slowly go clear to the floor with pressure. But it does not do that -- once the brakes are set, the pedal goes no further down.

The plot thickens.

Thanks for the help with this!

Frank

[hthorn]

Only pump I know of on those is next to the battery, for the suspension. That's a new one on me...
Next time you've started it up, pump the brake pedal several times after you've shut the engine off--then stand on it. (Literally!...) If it starts to sink after 30 seconds, the seals could be leaking.
The pump shouldn't affect the vacuum test, since its purpose is usually to supply additional vacuum at idle or under low vacuum condidtions. The test as previously described should not be affected because the conditions are changing too rapidly.

[FrankC]

Hi hthorn,

Thanks so much for the master brake cylinder test procedure. I'll give that a shot.

Here is a link to the vacuum pump if you are interested...
http://www.partsamerica.com/ProductD...pe=540&PTSet=A

I bought a vacuum gauge today, so I am ready to test for a defective cat. However, Midas Muffler today, at no charge, connected a pressure gauge ahead of the cat and measured back pressure. I was dissappointed to find out that they felt the cat was OK. They suggested that perhaps the timing chain had jumped a tooth. I do have a timing light, so I can check timing. They also told me that the 14 psi I measured should be more like 44 psi. I explained that the system uses a return line and that 14 psi isn't the direct pressure of the pump and that pressure in excess of that amount is allowed to release into the return line by a regulator within the TBI. Didn't seem as though they bought that.

So... it seems I need to check for vacuum leaks and to replace the wires, plugs, dist cap, and rotor. I'll also do the vacuum check for the cat.

Also need to buy a bottle of Lucas fuel treatment as mentioned by ehall.

I've learned a lot through this event! Thanks for that!

I'll let you know.

thanks,
Frank

[Edahall]

Your engine is TBI so 14 psi is correct not 44 psi. The newer cars with PFI use much higher pressures.

Sounds like your CAT is OK from the test that was done.

Next step should be to replace wires, plugs, dis cap, and rotors. Make sure you use AC Delco parts especially for the wires and plugs.

[FrankC]

Thanks Edahall.

I figured checking the timing would be an easy chore. I can't find any source for what the timing should be. I can't find the VECI label... perhaps it has fallen off. I did set it before. I can jumper pins A&B of the ALDL connector, but can't find what the timing should be.

Anyone know? 1986 Eldorado 4.1L, 87 octane fuel. I believe it should be 10 degrees but not sure.

Thanks!
Frank

[FrankC]

I just set the timing to 10 degrees. It was at 8 degrees. Then I checked vacuum which is pretty steady at 17 at idle. A quick snap of the throttle took the vacuum to 0, but it came quickly back to 20 then dropped to 17 when the throttle was brought back to idle. If I slowly opened throttle, the vacuum didn't change a lot and the engine sped up normally. When I snap the throttle, the engine hesitates - sometimes enough to stop running.

I was surprised to see the vacuum stay at zero with the engine off but ignition on ... I was measuring vacuum at the tee to the air cleaner. I figured the vacuum pump would have produced some vacuum. I am wondering if it is bad.

Also noticed that one of the two cooling fans was full on when the other one was still. Wondering if the fan controller is bad. Engine was not hot when that happened.

I haven't checked the O2 sensor. I believe it is on the rear manifold. But there is no trouble code about fuel mixture if that makes a difference.

It seems the next step is to replace plugs, wires, dist cap, and rotor.

[FrankC]

Thank You, Thank You, Thank You !!! To everybody that helped!

To summarize: Cad runs Can-Normal again!! No longer have to be afraid of going up inclines and such.

I replaced the fuel filter, but I am sure(?) it was OK. Also checked fuel pressure and vacuum, both of which looked good. I set timing to 10 degrees, which is what it is supposed to be, but the engine almost wouldn't start, so I backed it down to 6 degrees. Had exhuast back pressure checked and it was normal. Then I replaced the plugs, wires, rotor, and distributor cap. And the car runs fine. Seven plugs looked great. One plug (#6) looked as though it had not been firing. I have a bottle of Sea Foam for the fuel, which I will add next time I fill up.

It seems to me that it would have been more than one plug. Perhaps another one or two were intermittant(?) And should I assume it was the wire(s) or perhaps the distributor cap too. I did lose an ignition module just a couple of months ago and that was during the time the engine was running poorly. Perhaps the gaps between rotor and distributor and bad wire(s) cause stress to the ignition module(?)

I suppose I should bring the timing back closer to 10 now. Perhaps acceleration would improve a little.

Thank for all the help. I really appreciate all the support and guidance everybody gave. It was bad timing for me due to a couple of hernia repairs right in the middle of all of this. I wasn't supposed to lift and strain and such. And you can't own an Ht4100 engine without doing that at least weekly.

Thanks again!

Frank

[ehall]

Just in case... On the timing, you set it 10 degrees after you put the car into the set-timing mode. You don't just set it to 10 while the car is running.

Also, Seafoam does okay as a fuel system cleaner but it's nothing compared to the Lucas fuel treatment. Best use for it is to pull half the can through a vacuum line into the throttle body, and dump the other half into the oil, but you MUST change your oil and filter after 75-100 miles if you go that route.

[Edahall]

Glad to hear you got it fixed......

As ehall pointed out, you have to set the timing while in set timing mode. Read in your manual on how to do this. 10 degrees is what the factory recommends but you often can get by with a little more advance than that. Keep advancing it until it starts to ping just slightly at low rpm's / high power (lugging). This is the optimum point and you'll get more power and fuel efficiency. My car is set at 14 degrees using low grade gas. If I used premium, I would be able to advance it even more.

I think you mentioned earlier that you had aftermarket plugs and wires. That likely could have been the problem. A/C Delco plugs and wires are more expensive probably for a reason.

[FrankC]

Thanks guys for the tips.

I had planned to put the seafoam in today, but I will exchange it for the Lucas brand.

When I set timing, I did have it in the "set timing mode" or something similar on the display. I had jumpered pins A&B in the diagnostics plug. I haven't moved it up higher... closer to 10... yet. It just seems that there is something unusual in that the engine did not want to start when I had it at 10... granted I had the plug not firing. Normally, the engine starts within half a second. I had to crank it for perhaps 10 or 15 seconds to get it to finally start.... Almost as though the timing scale was off or the notch was in the wrong place (not possible). If it was off a tooth, however, I would think the engine would not run as nicely as it does. And it is just a 4 degree difference... (??) I'll move it up some today to approach the point of pinging as Edahall explains.

Incidently, the plugs I had were ACDelco plugs. And 7 of them looked perfect. The 8th one wasn't firing. But I don't think it was a plug problem, it was probably the 10 year old $22 set of wires or perhaps the original distributor cap or rotor.

Thanks again guys. You provide great advice in a great forum!

Frank