atreju

Hello everyone

My name is Atreju and I come from beautiful Switzerland. I own a 1987 Chevrolet Caprice Classlic Brougham with the electronically controlled carburetor. (Rochester 4bbl).

The car was in bad condition and I replaced almost everything at it, some problems with the engine I could not fix so far, unfortunately nobody could help me either.

Here's what I'm having trouble with:

jerking in the idle speed, unstable engine operation, stuttering
Jerking at constant speed, especially at about 50 miles per hour, becomes better immediately if some throttle is given.
Swallow more briefly when changing from stationary gas to low gas, for example when starting up.

Almost everything has been replaced, unfortunately the problem still exists:
Completely new ignition with distributor, ignition control unit, ignition relay, ignition cable, plugs and engine control unit
New timing chain
New thermostat
map sensor

And much more, which probably does not contribute to the problem.

What else could be the problem? I'm getting nowhere.

Any of you know of a solution?

Greetings and thank you

Here a video - maybe it helps

navy02ws6

If you haven't already, I'd recommend a full rebuild of the carburetor. Check for vacuum leaks, lots of vacuum hoses in those cars.

atreju

Thank you for the hint.

Unfortunately the carburetor is completly new (rebuild of rockauto.com) - it was one of the first things I replaced. I checked all Vacuum hoses and am not able to find a leak.

gotmud7

Did you adjust the TPS voltage? The new sensor may not be calibrated.

atreju

Hi - no I did not adjust the TPS voltage. is there a manual for the correct values?

Thank you

Kind regards
Simon

midias

Here you go

https://www.thirdgen.org/forums/carb...e-rebuild.html


https://www.thirdgen.org/forums/carb...rochester.html

Schurkey

I'm sure that complicates owning an "American" car.



You will want an OBD-I scan tool that can read the data stream, and the appropriate adapter to connect to your vehicle.


If that car was in MY driveway, I would do a compression test of all eight cylinders. If any of the cylinders were low, it'd get a leakdown test as well. NO engine will run properly with leaking valves, or worn-out rings.


I would perform a cylinder-balance test to locate weak ("jerking") cylinders at idle. If every-other cylinder in the firing order is weak...you've likely got a problem with the idle fuel mixture passages in the carb. If one or two are weak, you may have an air leak into one or two runners of the intake manifold--look for vacuum tees screwed into the runners, and trace those vacuum hoses.


When you determine that the engine is in acceptable mechanical condition (good compression on all eight) you can move on to the fuel and ignition systems, and look at the data stream to see if there are issues with sensors, actuators, and the ECM.

Have you verified that all sixteen valves are opening about the same amount? Cam-lobe problems are fairly common.
What is your fuel pressure when the engine is running poorly?
Have the tamperproof plugs over the idle mixture screws been removed? Have you adjusted the idle mixture?
Is the timing set properly? Does it advance like it should?
Knock sensor work?
Coolant temperature sensor reporting accurate data to the ECM?
How old is the O2 sensor? Does it produce reasonable readings including cross-counts? (sensor is not "lazy"?)

EGR valve seal at idle so that exhaust gas doesn't contaminate the fuel/air mixture at idle?
Does the choke blade shut when cold, and open at the proper speed? A properly-functioning choke will allow SEAMLESS starting and operation of the engine at any temperature from -20F (or colder) through 230+ degrees F.

atreju

Sorry for the long time I did not answer.

It took some time to organize the tools and so.

I measured the compression and got (cold and without extra oil in the cylinder) the following values:

midias

What is your base timing? How did you set it?

atreju

The timing was extremly difficult to set because of the bad idle, the timing is hopping up and down.

I dont know the exact word for the tool in english - a Stroboscope light?
A pistol which has one connector for the 1th cylinder spark wire, and then flashes to see the mark on the harmonic balancer and the marks on the engine block.

I set the timing with A and B bridget on the ADLD connector and with the 4pin connector unplugged on the distributor. I tried both ways.
And I set the timing to 8 degrees before OT.

But I can change the timing to whatever I want - the bad idle does not getting better.

kind regards

midias

Are you sure you have the distributor wired correctly? I have seen issues like this where a couple wires are swapped on the distributor. Even after checking two or 3 times the error was still not caught.

atreju

I checked the firing order and wiring dozen of times and also other person. But thank you for the image - I will check it again with this picture.

But if I had wrong wiring, I thought the engine would not run fine when a little bit of throttle is given?

Kind regards
Simon

midias

You are right the engine should miss with a swapped wire at all RPM.
Have you checked fuel pressure?
If you spray some starting fluid down the carb while it is running does it smooth out?
Have you tried spraying staring fluid around the intake looking for a leak?
If you put cylinder #1 at top dead center and pull the distributor cap is the rotor pointing at cylinder #1?
Did you ever adjust the TPS voltage?

atreju

I did not check fuel pressure - but the fuel pump is also new and I have the same issue.

I need to check what happens when I spray directly in the carb.

When I spray around the intake manifold, nothing happens. I also changed the Intake Manifold Gasket and checked all vacuum hoses.

Yes, when I put cylinder 1 in TDC, the distributor is pointing no no1 cylinder

GM B-body

Hey Atreyu, I own a caprice myself and am glad to help you find the issue. How can I help?

atreju

Thank you for your assist.

I'm at the end of my knowledge. The big problem is, that nobody in Switzerland really knows the old american cars. Of course we have really good mechanicans and also a lot of people who know how to fix a car with carburetor. But US cars are different from Japanese or European cars, especially the elecronically controlled carburetor and the tons of vacuum hoses.

So in theorie the car is not difficult to fix but we have symtoms which does not make sende and almost everything on the car is new but it runs most of the time like crap.

I have:

- Rough idle, the car is jerking bad, is getting way worse when hot
- I have bad power output at kickdown, sometime shuttering
- I have idle rev problems, sometime its idle way too high and sometimes its dying from itself.

Then i have black clouds coming out the exhaust when accelerating but no oil comsumtion, so I think its running too rich most of the time.

But all is new, carburetor, ignition, ecm, all stuff with cooling and temperature, timing chain.. and none of these fixed the issue.