Greetings from germany,

I usualy just read things here, but I ran out of ideas with the problems of my dream car that I bought last year.
I own a 1987 305 Iroc-Z TPI

It always was running with a high idle, so I started to fix it, with limited success....maybe some of you guys have any more ideas what to do. In Germany close to noone got experience with theese cars

I always idles around 1200-1500rpm (read from tachometer)

I did the following things (ofc I changed all fluids)

• Cleaned the throttle body
• new IAC
• new coolant temp sensor, and new thermostat
• adjusted the TPS
• new Spark Plugs, Plug wires and a new Distributor (whole assembly)
• searched for vacuum leaks, found none, changed the lines anways
• new gaskets for Throttlebody, Intake manifold
• checked timing (without EST its 7 degress advanced, at 6 it kinda pings or stalls)

When I let it run without EST and at 7° its idles around 900...but sounds like I have biggest cams imaginable ( stock cams).
When I plug in the EST again, it surges around 1100-2000+....and I think it runs very very rich

Anyone have some ideas? I'm getting kinda depressed here

 

Welcome aboard!

A very important item to remember is that there is no tachometer in these vehicles. There is an indicator which moves as engine RPM changes, but no one has ever been able to determine what the numbers on its scale represent.

In other words, the "tachometer" the factory installed to fill that space in the instrument panel might have been accurate once, for about a year or two, but is probably now very far out of calibration. It is crucial to use a separate diagnostic tachometer whenever analyzing or adjusting the system.

Once the RPM can be checked reliably, perform the minimum idle speed adjustment once again, and the readjust the TPS voltage as necessary.

The base timing at 7° should be reasonable, and shouldn't change once the other symptoms are resolved.

With the other work you performed, including replacing spark plugs, plug wires, and the complete distributor, there were several opportunities to get the plug wires installed incorrectly. With cylinders 5 and 7 being both physically adjacent, and sequential in the firing order, it is VERY easy to inadvertently connect those wires to the wrong plugs. I've done it myself enough times to know that's one of the first things to check. That might also explain the "big cam" impression at idle, as well as a rich condition from unburned fuel.

 

If you are curious about the tachometer issue, this may be of some interest: https://www.thirdgen.org/forums/elec...-tach-fix.html

 

thank you very much for the hints.
I will check the wiring again and get myself a tool to read the rpm more precisly

I making small progresses...it idles still rough but after the cold start the rpm drops to 500-600 (again read in the dashboard) but wants to stall and surges up as if its fighting
I will get back on the hints from you asap

 

Just wondering...the whole purpose of me trying to fix the apperently rpm issue was that the "buck" when you shift from P to D was kinda rough and I thought that it might be caused by the high idle....
Is that so?, does your cars also buck a good amount when shifting into D?

 

My '86 TPI/TH700-R4 does have a noticeable engagement when shifting into either reverse or a forward range. The ECM commands a higher target idle RPM when the gear selector is in PARK, and the system does not directly control transmission line pressure as is the case for later, more modern ECM/PCM systems. As a result, drive engagement will never be anything like a more modern vehicle where the target idle is temporarily dropped, line pressure is temporarily lowered, and activation of transmission solenoids is delayed or PWM "softened" to reduce the harshness of engagement into a range. Understand that later torque converters also have a lot less pumping in the lower RPM ranges (mostly because they are smaller), and are therefore "softer" to drive the turbine of the converter. Powertrain engineers have wrestled for years to make an automatic feel like it is never even shifting, which struggling to make them last without slipping and burning themselves into junk. Prime examples are the 6L80E, 9T65, and other boxes with more ranges than power handling.

In short, having to apply the brake to keep from moving when the transmission engages with some real authority is not only normal, but somewhat of a good thing.

 

A higher stall converter helps with that. My 2500 rpm converter engages A LOT easier than my old factory converter.

 

Yes a high idle speed will cause a clunk when engaging any gear. is the throttle plate base idle speed screw set correctly? It is on the throttle plate covered by a plug from the factory. I read all that you did including searching for a vacuum leak. Air low into the engine is what gets regulated by the ECM for idle speed. there used to be 2 idle speed specs on tune up decal under the hood. You would have to unplug the IAC motor to get the throttle closed enough to set the base idle and timing.

 

Thanks for the replies.

The little buck when shifting into D isn't annoying for me I was just wondering if its damages the transmission.

@Gordon, yeah i Did that also.

• removed the small cap
• screwed it back to let the blade close all the way and have just a little tension
• and the IAC was also unplugged
• turned it down to like 400-500 RPM (reading from tachometer)

I Just ordered the parts to build a "OBD" cable and use WinALDL to get some exact reading done.

The car currently drives fine beside that it feels like a have no engine break at all and I can kinda cruise with 35mph without hitting the gas ...and it does not want to the stop and idle (it starts to buck after 10 seconds like crazy)

 

A scan tool is not necessary. A plain, commonly available diagnostic tachometer will be sufficient for setting minimum throttle position.

The lack of engine braking is not only common on '86 and some '87 model years, but was actually part of the list of symptoms for the TSB (86-162 / 86-162R) issued regarding the replacement CALPAK offered to resolve that and some other nuisance issues. My '86 would idle at 35 MPH due to it not unlocking the TCC until braking so the speed reduced to less than 27 MPH.

 

Update

The car runs drives now just fine....after all the stuff i've done I'm starting to think that it was the IAC Valve....

Today I unplugged it after resetting it (A-B Pin ALDL) and measured the idle rpm and set it to around 600...

I remembered that someone in the forum wrote that he had to tighten the IAC valve really hard to get it to sit properly...so I tightened it to the point where I hope I never have to open it again...(maybe another 1/10 of a revolution)

After that the cars was running like **** for like 5 minutes...shut it off, let it sit for 2 mins and after that the car was idling rock steady and drove just fine

Thanks to everybody who replied (I have a feeling it wasn't the last time I asked something )

ps sry for the bad english I'm german

 

It is good that you were able to set the minimum idle air, Driving the car a bit longer may provide more understanding of its behavior, like holding a high idle until it's nearly stopped moving, the transmission shifting conditions, cold starting habits, and other quirks,

This forum has been here for 25 years. Someone will be around here if you find more questions.

BTW, your English translations are just fine - Better than a lot of American English posts we see.

 

What brand IAC did you use?


I just learned from another thread that there are apparently 2 units... 85-89 and 90-92. Standard motors has them.
Im assuming the ones that list 1 unit for 85-92 are incorrect units. The Standard motor one i got for an 87 was "AC1"



I used a scan tool and noticed the ac delco unit I had before had counts of i beleive 140 or so at all times. The new one changes and seems to regulate the idle much better. Stil lin testing / adjusting phases though