Plenum Removed - Now No Secondaries

[Bob Eyres]

We took the plenum off my 91’ to replace the starter. Now the new starter works fine, but the engine falls flat over 3K rpm. We checked the vacuum pump and it wasn’t running at start up. Replaced it, vacuum is good, but still those secondary butterflies won’t open. Got to open the plenum again. What’s the likely culprit?
BTW, the induction has been working fine in it’s stock configuration for 28 years previous to this plenum pull.


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[lfalzarano]

Check that the black vacuum line leaving the passenger side of the plenum going towards the radiator and check it didn’t separate during the removal.

If not, a member who has pulled the plenum will guide you to a solution.


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[Marc Haibeck]

Hi Bob,

If the vacuum pump turns off for at least a second with the key on and the engine not running the vacuum up to the inlet of the secondary vacuum solenoid is good. The 3000 rpm rev limit is imposed if the vacuum system fails the diagnostic test when the secondaries are activated. Code 61 will set if the vacuum fault persists for over three seconds. There is a vacuum fault on the outlet side of the vacuum solenoid. Activate the secondaries with the plenum off and it should be easy to track down the cause. Use this procedure:
http://www.zr1specialist.com/HAT%20W...tle%20test.htm

[Bob Eyres]

Thanks Marc, we will do that procedure today.

One more thing, I was surprised by the way this problem showed up. I thought that if the secondaries somehow failed, the engine would behave as if the power key wasn't turned on. Instead the motor will idle fine, and be driveable
up to 3K rpm. Then it will lose power, missing repeatedly, and be undriveable over 3K.
With these symptoms, what could be the possibilities? Secondaries partially open? Or something else is failing?
Just to re-state, this happened after putting a new starter in. So I knew we screwed it up somehow as these secondaries have been operating perfectly for 28 years without fault.
I'll post later to tell what we find.

Thanks.

[Bob Eyres]

Today we checked the secondaries with the plenum off acording to Marc’s instructions. They seem to be functioning OK.
The butterflies opened all the way on both sides, although they didn’t seem to pop open quickly like I expected.
We then re-installed the plenum and road tested.
It still starts and idles smooth, and after warmup I took it through the gears. This time it made it smoothly up to 4,500 before it stumbled again and power fell off (all with power key on).
Up until this problem it would pull smoothly from 2K to 7K in all the gears Without stumbling.
Today, in any gear it would build speed slowly, but if you put the pedal down all the way it would fall apart, missing badly.
We strapped a fuel pressure to the windshield and it registered 40-43 lbs. at idle a built pressure as revs climbed to 50 lbs. with the hammer down and bad stumbling.
This feels like a fuel problem, what should it be reading?
We’re going to check the fuel pumps tomorrow. Stay tuned.
Any thoughts are welcome.


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[RussMcB]

My initial (non-professional) thought is that fuel pressure sounds fine.

Is there anyone nearby with another ECU to swap in (just to rule it out)? That thought occurred to me when I wondered if the secondary injectors might not be opening up.

Good luck. Sorry you're having to deal with this. Hopefully it will get fixed soon.

[jss06c6]

I would strongly suggest you recheck all of you vacuum lines. When you turn the key on, does your vacuum pump run for a few seconds the shut off? If your vacuum lines are all in great shape, you shouldn't hear the vacuum pump come back in for at least 10 seconds.

If this part is good, check Marc's tech bulletin for forcing the secondaries open with a jumper wire at the ECM.

Check both of your vacuum lines at each MAP sensor too..

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[dredgeguy]

Is it possible the air intake duct is collapsing at higher RPM? Does it have a stiffener inside?

[Paul Workman]

Originally Posted by dredgeguy

Is it possible the air intake duct is collapsing at higher RPM? Does it have a stiffener inside?

Um, yeah...you may be right. But, the problem began only AFTER the plenum was pulled. 'Course; never assume! If the hose is not reinforced up to this point, maybe NOW would be a good time to do that. (At least THAT potential problem would be eliminated!)

[Bob Eyres]

The air intake duct is OK. I put a Coplon stainless sleeve in there 20 years ago.
I put two fresh fuel pumps in today. I had a feeling that’s what it was. NAaaah, it now still runs fine up to around 4K, then cuts out repeatedly like before.
I’m going to go over all the rubber vacuum lines next, then check the fuel pressure regulator. Any thoughts on that?
I still think we did something stupid when first pulling the plenum.
Thanks for your thoughts guys.


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