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Old Yesterday, 09:33 AM   #1
Tollroad_Playground
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Join Date: Jul 2018
Location: Dallas
Posts: 7
Not the normal idle issues

Iím new to E30s and DIY on old cars so I really tried to read threads and do a lot of normal fixes before posting. You guys are smart and busy. But this issue seems unique to all the idle problems Iíve been reading about. Maybe some big brains out there can crack the case.

I bought the car with 165k and have added 5k. It was very well kept and runs strong. Since Iíve had it the idle has always been just above 500. When the car warms up, it starts to drop and catch itself a bit making the engine shake enough to feel it a little inside the car. Not a ton, but itís there. Sometimes at lights the check engine light will come on after a shake. A quick rev will turn the light off. An off idle throttle blip drops the RPMs initially but it recovers quickly and will rev perfectly from there. Gas mileage has remained at 20-21mph no matter how I drive. Iím new to older cars but the idle exhaust seems rich. No black smoke or anything extreme. Just reminds me of the smell and sound of an old American muscle car. Turning the AC on temporarily raises the idle and smooths out the engine but it goes back to 500-600 quickly and gets rough again. Iíve read a lot about idle that revs up and down or that is too high, but this seems totally different.

Hereís what Iíve done to chase the issue:
- New paper air filter
- Removed air box and made sure nothing was dirty or stuck
- Removed all rubber going to the throttle body and checked for leaks
- Replaced one hose under the TB from 1988 with a split
- Cleaned the TB and ICV really well
- Checked to make sure the ICV buzzes
- New plugs. The old ones were well worn but perfect brown
- New rotor and distributor cap

Also did some non-related maintenance stuff:
- 50/20 oil change
- Radiator flush with 50/50 blue coolant and distilled water
- Replaced fuel filter

None of the intake cleaning and repair changed anything. The new distributor/cap/plugs made it run smoother and added some nice low end torque, but the idle issue is exactly the same.

Thereís one other thing that might be throwing it off: The car was converted from automatic to manual with a rebuilt stock 5-speed. It had cruise control that the seller said stopped working after the conversion. I wonder if engines with automatics had a different idle setup that has trouble with the manual clutch. As soon as the clutch is engaged and I creep along with no throttle it calms down.

I plan to do a full interior restore and drive this guy all the time. Iím willing to invest in making it right for everyday use. The people Iíve brought it to so far either donít work on old BMWs or they rig up old stuff and tell me I should be lucky it starts every time. So I turn to you! Help me get this fella running like it should.
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Old Today, 04:59 AM   #2
tomstin
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Join Date: Mar 2015
Location: Wake Forest, NC
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First, what engine?

As for the rich idle, check the O2 sensor. My other thought on the idle, would be to check the throttle position switch located under the throttle body.
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Old Today, 06:42 AM   #3
Tollroad_Playground
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M20/25

TPS was my next move. Kicking myself for not looking at it when I had the throttle body pulled. I donít have a multimeter or any skills with electronics. Any easy way to test it before replacing?
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Old Today, 06:54 AM   #4
sert57
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Join Date: May 2014
Location: Georgia
Posts: 134
Sounds similar to what my car did when it had a vacuum leak. Since you said you removed all rubber/hoses to the TB I assume you checked the intake elbow for cracks. One difference between automatics and manuals is the cover on the intake manifold (P/n 11611708858) where automatics have a vacuum take off and manuals do not. Check to make sure this was plugged or swapped or something. It could be an easy thing to miss when doing a swap.
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Old Today, 08:43 AM   #5
325e '87
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Join Date: May 2017
Location: SC
Posts: 328
I’d say it wouldn’t hurt to get a new 02 sensor.

You’ll need to test the throttle position switch. Or just get a new one. People drill a 1/8” hole in the center of the new ones to let oil drip out, its just oil vapor from the valve cover that collects in the throttle body and drains down the switch shaft.

A new coolant temp sensor would be good if the one there is old looking. The 2 pin one is for the fuel injection system, single pin one is for the dash gauge.

Vacuum leaks are easy to overlook. I can make my engine idle like you describe buy pulling the dipstick or loosening the oil fill cap on the valve cover. My last leak was at the brake booster fitting where there’s the vacuum line on the elbow on the booster.
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Old Today, 01:34 PM   #6
Tollroad_Playground
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Join Date: Jul 2018
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Thanks so much guys.

I'll look into the vacuum takeoff on the intake manifold.

With 170k on the ticker I think a new O2 sensor and TPS wouldn't hurt as well. I'll look at the coolant temp sensor while I'm at it. Trying to do this with fun money at the end of each month.

If I pull the dip stick or open the oil cap the engine bucks and chokes much harder with wider swings. My typically low idle seems to stay on the edge of staving off the air all the time. That intake manifold change might be the silver bullet.
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