blownrunner

I have gotten my 91 zr-1 back together after changing injectors, wires, plugs, fuel filter, oil and filter change, cleaned throttle body, plenum and injector housing interiors, new jerry's gaskets, new air filter, new fuel filter, set TPS to .53 volts when fully closed, among other things. Just got the car and it needed a little work.

Anyway, it is back together and it idles I guess at about 800rpm. When I drive it and get the rpm's up to 4-5k, then push in the clutch, many times it will stay at 3k rpm, even when completely stopped. It will stay this way until i rev it again and then the rpm's will go down to normal. Sometimes it will fluctuate from 800-1200 or so at idle.

The throttle body was removed and I completely cleaned it, installed new gaskets, and torqued to specs. I removed the TPS and IAC when I did the cleaning, put a light coat of grease on the rubber of the IAC and set TPS to .53 with TB fully closed.

It seems like something is stuck open, letting air into the engine. Could the IAC be bad/stuck open? I read that there is some type of grease on the IAC valve, so after I cleaned it I put a small dab on it.

Since doing all the work I did, it has a smooth idle when it doesn't act up, I just have this one problem. It is like driving with cruise control on, and the RPM's increase when i push in the clutch. It seems to mostly happen when I really push on the gas pedal, if i go easy on it, it seems to be normal. Need some help with this one.

Mr. Gizmo

It is either the TPS(Throttle position sensor) or the chip. I would bet it is the tps. when it is idling high, adjust it manually and see if that helps.
When I pulled the plenum on my 91 c4 zr-1, because of the secondary actuators being installed backwards from the factory and the butterflies weren't opening- when i got it back together i remember having this problem. if that does not fix it you might buy a new tPS-- i would imagine you can find one relatively inexpensive.

There was also a recall on the chip as i recall back in the day that caused a similiar problem for 91's. Check on the zr1 registry. they have info on this problem over there.

good luck solving it.

LT5corsa

You mentioned "cleaned throttle body" and I remember you need to be careful how you clean the TB

Have you seen the club new website? www.zr1netregistry.com ? Lots of helpful info there and great forum with hundreds of ZR-1 owners to help



David

Dominic Sorresso

Also what are your IAC step counts?

Dynomite

Also.........Lots of related helpful info here in this Forum C4 ZR-1 Discussion under these Searches:
IAC issues
IAC fixing
IAC on TB
IAC on throttle body

Fully Vetted

Seriously? From the factory? That's actually kind of funny. I mean, I'm sure you didn't think it was funny at the time but just hearing of something being installed backwards makes me chuckle a little. I guess someone had one too many Margaritas at lunch that day.

Paul Workman

It sounds more like something sticking in the TB; either the throttle plates binding (did you remove them when you cleaned it?), OR the IAC binding.

You already checked the TPS (@ 5.3 VDC) which is about dead-on at idle. And, because it will idle sometimes, especially after you blip the throttle, I'd not be thinking plenum gasket.

I did have a TB gasket problem once, resulting in high idle, but it did not respond to blipping the throttle, of course - and therefore I'm nearly convinced it is something binding.

Let us know...

Oh, and Dom is right about IAC counts. If there is something sticking, counts will be very low - something like less than 5 (0-1,2?) while the idle is "up there". I'd bet a ham sandwich its TB related.

P.

WVZR-1

The old DAG compound hasn't been mentioned but he seems to have all of the symptoms and has just "cleaned" the throttle body SO might that be the next thought!

http://www.zr1specialist.com/HAT%20W...tle%20body.htm

blownrunner

What is an IAC step count?

blownrunner

I was thinking that the throttle body plates were sticking also, removed the throttle body and I cannot get them to remain open/stick. Should I be able to move the IAC valve by hand? It seems to be in a fixed position when I removed it. I also had the RPM's fluctuating between 800-1200 every few seconds, maybe the IAC is bad.

blownrunner

What is this chip you are talking about? Thanks.

A26B

In actuality, this is not accurate. Both the Assembly Manager and the Engine Building Supervisor are good friends of mine. The truth is: installed with the actuator arm in line with the linkage is what caused a problem. The problem is the arm sits to one side of center due to an "L" bend in the arm for attachment to the actuator diaphram.

They noticed the binding/dragging problem & turned the actuator over. Now the center of the actuator diaphram lines up with the secondary linkage and no more binding, less vacuum, quicker & smoother opening.

When the actuator arm is turned to straight alignment it doesn't work nearly as well. I know this from personal experience.

Fully Vetted

That makes more sense than being installed backwards...

Dominic Sorresso

It's probably not the IAC. You need to make certain that the Minimum Air Adjustment is done. Then check TPS voltage at Key On. Finally, we're the throttle blades removed? If so, they need to be properly positioned in the throttle bores.

combatdoc

Erg? I realize tuned port injection is not what an lt5 is, not even close but I had the same issue with a tuned port motor a few years back and right before I pulled all my hair out I changed the egr and would'nt you know that solved the issue!

blownrunner

Thank you everyone for all your help. I found out what is wrong with it!
Even though it is not fixed, finding out what is wrong with it is the hardest part of the repair sometimes.

I replaced the IAC valve and that didn't change anything. At first glance, the primary and secondary valves in the throttle body appeared to be closed. After revving the engine, RPM was 1200 on the tach, then it cycled from 800 to 1200, which I assume is the IAC trying to control idle RPM. I pushed on the secondary valves through the front horn and they closed about 0.010" or so and the engine stayed at a perfect 800 RPM. Yes, the DAB (or whatever it's called) I removed when I cleaned the throttle body. I didn't disassemble the throttle body, but the throttle body valves could be misadjusted or have worn parts, or require replacement return springs.

On a side note, I met this older guy named Larry who owns a 1988 prototype ZR-1 that was used as a test vehicle for GM, that was stored at GM with the engine removed for many years until it was bought by someone instead of being destroyed. Larry bought it after the owner over many years could not sell it because "yeah, right. GM never built a 1988 ZR-1". He put an original 375HP engine in it and I believe it is somewhere in a showroom or museum. He doesn't keep it at his house/garage because he wants the public to enjoy it. The original build sheet has all the options crossed out and all the prototype changes hand written in. So I call up Larry and he is at the corvette NCRS show in Orlando this Friday night to tell him the good news because I met him a few weeks earlier at a show where we talked for well over an hour when he saw my car. He is Mr. NCRS for the area and is running the show, like the president or something. He says to me over the phone "I have a brand new throttle body for the LT5, I will let you swap it out on your engine and if it cures the problem, you can borrow it while we send your throttle body to some specialist guy to get it rebuilt/repaired!

I am going to meet up with him tomorrow Saturday at the show he says there are tons of vendors in Orlando along with 150 corvettes in the show. Only thing is I will have to take my daily driver, not the ZR-1. Maybe I will have it ready for next years show.

Mr. Gizmo

Yuppers - they were on backwards, the butterflies would not open on the drivers side bank and would open about a 1/4 of the way on the other side. the car would fall flat on its face at 5400 rpm. A neighbor of a friend of mine had a red 1990, and his had the exact same problem

There was a woman on the Lt5 engine assembly that was putting these actuators on bass ackwards on the later produced 90's and earlier 91's. -- Mine was a 91. ---. or at least that was the rumor going around on the team zr1 list server back in the 96 time frame. I bought my 91 used in 96 with just a few hundred miles. prior owner was a collector, and don't think he ever turned the power key on when he drove it.

Dominic Sorresso

Send the TB to Marc Haibeck. He will properly position the throttle blades.
He can also install a helper spring to make sure the throttle shuts properly. I did this with my throttle body last year after disassembly.
However, you still may need to set Min Air once on the car. Make sure you install a hex head bolt for the Min Air adjustment before assembling onto plenum. That way you fine adjust the Min Air w motor running. You'll need a scan tool also.

A26B

I'm sorry, but that story just doesn't ring true. First of all, EVERY LT5 engine that left Mercury was dyno tested before shipment. Any reasonable person would realize that if such an engine would not have met the minimum power standard and been rejected and sent back to assembly or junked.

Secondly, I personally know several of the girls who worked on the LT5 assembly line, Scott Skinner the engine assembly instructor and Chris Allen, who was in charge of the entire LT5 assembly line development and process. The LT5 was essentially a hand-built engine produced to race engine tolerances, by Mercury employees who filled voluntary positions with no guarantee that the positions they were leaving, earned by time & performance, would be available to them after the LT5 program was completed. The LT5 team was no half-azzed bunch of yokles.

Finally, in a practical sense, from my personal experience and considerable conversations with real experts about this rumor, not one instance of so-called "backward installed" secondary port throttle actuators has ever prevented the secondaries from operating as you stated, There are other factors that may have caused the symptoms that you describe, but it is not because As stated in my post above: Your post about "a woman" doing sub-standard work is unfounded and unwarranted. To further promote baseless rumors only serves to promote mistruths about the LT5 and cast a bad light on individuals who consider the LT5 program at Mercury Marine in Stillwater, OK, the highlight of their entire career.

Mr. Gizmo

Please don't allow your undies to get in a bunch over this.

This was a problem on the car when I purchased it and it was that way from the factory. The guys on the teamzr1 list server at that period of time helped me solve the problem as the car was out of warranty when i bought it. Could lack of use also been a contributing factor to the problem -- Maybe. But after removing the plennum and simply spraying carb cleaner around the butterflies still did not allow them to operate 100% freely.

You are obviously aware of the problem. -- I would agree that it is not as serious as these Ls7's dropping valves and windowing blocks - but a problem non the less and frustrating to me at that time.