Noob in need of guidance

[bsrjironman]

New guy here. After much patience and monitoring classifieds, I finally [towed home] a 1980 300cd N/A this past weekend, for a deal.

I’ve been working through some of the issues and have got it running pretty well. Fixed numerous vacuum leaks (or isolated faulty networks), replaced both fuel filters, adjusted valves, cleaned and freed all calipers, etc. Anyway, Here are issues that I am having a little more difficulty dialing in:

-shift points are uncomfortably high. Relevant data points include; 15hg @ modulator. No Bowden cable equipped. Fresh fluid. Overall vacuum reading @main manifold tube is 22hg steady.

-low power @ high speeds. Almost like it reaches some sort of internal limit at 45 mph. Even at WOT it cannot accelerate beyond 45. New filters have remained clean after test drives. Relevant data points; New clean filters. ADA dialed out. Linkage adjusted and sound. My next suspects are rack limiter adjustment, and tank screen.

I’m also suspecting the white vacuum valve bolted to the top of the valve cover. There seem to be two positions, and they seem to be dependent on throttle position. Not sure what this is called. Can someone Identify this mysterious component for me so I can study it?

Well it’s the end of the day, and my mind is dulled. Any insight to be offered to a fledgling enthusiast?

[BillGrissom]

Sounds like a lot of productive work in a short time. You say "linkage OK", but did you verify the rubber isolator at the firewall isn't split? That happened to me driving home long ago when it wouldn't go above an idle and I thought I must be running out of fuel.

[Jarod]

Considering I've just been through a bad diesel clogging every filter in 2 cars adventure, I'd aim at the tank strainer first. you could even hook a small pump to the fuel line at the front of the car and tee in a vacuum gauge to see how much flow you get and how much vacuum is pulled to achieve said flow.

Did N/A models not have a bowden cable? I know vacuum on my cars only controls the shift firmness, the points are controlled solely by the bowden cable and road speed.

[Maxbumpo]

The easiest tank strainer test is to reverse the fuel supply and return lines in the engine bay, so the lift pump is sucking from the return line (which has no strainer) and returning to the supply side. Keep the fuel level in the tank up - the supply is at a slightly lower point than the return line, so if you reverse the lines, your car will appear to run out of fuel sooner than normal. Some have reported this to be when the gauge is still reading 1/4 tank or higher.

Don't forget to put the lines back in order after the test is over!

[Maxbumpo]

A weak lift pump might cause power loss at speed? Try clamping off the cigar hose (return line) and see if there is any improvement.

[Mxfrank]

Transmission vacuum control valves:

Transmisson Vacuum Valve Lever Replacement

[t walgamuth]

If the strainer in the tank is suspected I'd use a biocide before doing anything else. If the tank is infested with bacteria it will kill them and they'll come through your filters without further incident, though a second dose might be needed. Bacteria will grow in your tank if water is present in the fuel.

[pj67coll]

Originally Posted by BillGrissom

Sounds like a lot of productive work in a short time. You say "linkage OK", but did you verify the rubber isolator at the firewall isn't split?

This. Woludnt be the first time the firewall thingy was the issue and being pegged at 45 mph sure sounds suspiciously like the full travel of the accelerator isn't making it to the pump.

- Peter.

[barry12345]

Originally Posted by pj67coll

This. Woludnt be the first time the firewall thingy was the issue and being pegged at 45 mph sure sounds suspiciously like the full travel of the accelerator isn't making it to the pump.

- Peter.

Really takes two people usually to determine. One to press the go pedal and another to monitor and verify that the arm on the injection pump hits the stop.

[bsrjironman]

You say "linkage OK", but did you verify the rubber isolator at the firewall isn't split?

Yes, I've verified that the pedal input produces complete range of motion of rack (per current adjustments), with "acceptable" deflection of pivot components observed. The rubber isolator (on firewall) is showing age, but is intact and functioning.

Did N/A models not have a bowden cable? I know vacuum on my cars only controls the shift firmness, the points are controlled solely by the bowden cable and road speed.

I'm uncertain which models were not equipped, but I have learned that "some" were so configured.

The easiest tank strainer test is to...

That's not a bad idea...

A weak lift pump might cause power loss at speed? Try clamping off the cigar hose (return line) and see if there is any improvement.

How exactly does this decrease strain on the pump? Logic seems to dictate that obstructing the fuel path would increase load?

Transmission vacuum control valves:

THANK YOU!!

If the tank is infested with bacteria it will kill them and they'll come through your filters without further incident

You're suggesting that if the microbiota are getting stopped at the strainer, killing them will cause them to pass through the strainer, the primary, and the secondary filters? Should this be preferred to exterminating them at the source?

being pegged at 45 mph sure sounds suspiciously like the full travel of the accelerator isn't making it to the pump.

That's (roughly) along the same lines of my current theory. After inspecting the linkage, and range of travel, it seems [to me] the the rack is being limited earlier than necessary. In fact, some of the linkage (including the Transmission vacuum control valve!!) has been "adjusted" previously. There is excess travel and wasted movement in various places. In the following picture, this limiting adjustment (RED ARROW) is set so as to allow the least amount of travel (the bolt is 'backed out' approximately 80% of the way.)

I've yet to find the proper adjustment procedure for this particular configuration, but am now suspecting it as a possible cause for my lack of power at speed. Anyone with insight regarding this proposal? Thanks a bunch for the responses thus far!

[bsrjironman]

After further review of the data, I no longer suspect that the above pictures arm travel is the [main] culprit. WOT position events should be rare during normal driving conditions, therefore the full-range would rarely be utilized. In other words, the car should be able to achieve 45mph within partial range, and the fact that this may be adjusted incorrectly, is moot at this point.

[Diseasel300]

When you're at 45mph what gear are you in? If it's already shifted up to 4th and isn't downshifting, you're not going to move very fast with 79HP in a 3500 pound car

[bsrjironman]

Originally Posted by Diseasel300

When you're at 45mph what gear are you in? If it's already shifted up to 4th and isn't downshifting, you're not going to move very fast with 79HP in a 3500 pound car

That's a good point. It may behoove me to get the transmission operating properly, and then re-assess the power conveyance. It was, in fact in fourth gear, and as the shift points are very high, it doesn't readily downshift.

But aren't these cars capable of freeway speeds? This vehicle will not go beyond 45mph, even @ zero incline. Seems to accelerate to that point just fine.

[bsrjironman]

My current theory is the following (hydraulics 101 ):

higher vehicle speeds = higher fuel demand (@pressure constant) to effect otherwise equal acceleration rates. (net increased overall fuel system demand load)

This leads me to suspect, again, the tank screen as a possible restriction at speed. The only visible cue I can think of, would be to inspect fuel vacuum, across the demand spectrum. Or, as Maxbumpo suggested, reverse fuel lines (temporarily).

[Dubyagee]

Or a stand alone fuel source.


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