Hi
Posted a while back about getting the 88 745 smogged (Ca). It failed. Had the cat replaced, cleaned throttle body, replaced cap & rotor, replaced wires, replaced plugs, replaced O2 sensor. Car started fine but had a fluctuating idle. It would go from around 725 to 800. Replaced the IAC with a doner from my parts car. I cleaned and checked it before replacement. The idle smoothed out, not perfectly steady, but dang close. The trouble is after replacing even with the idle adjustment knob on the bottom of the TB screwed completely in I can not get the idle below 800rpms. What should I be looking at next? The temp sensor or is it possibly just a vacuum leak that I haven't found yet?

Thanks Brad

Hi
Sorry for the late reply. Finally got the car smogged today and it passed. Thanks everyone for the help.
Brad

Ok Spent the morning checking out stuff. Here is what I did and the results.

First warmed up car. Hooked up tach and timing light.

Removed electrical plug from TPS and restarted car. - result engine speed increased.

Jumped tps switch wire loom plug center pole with side poles one at a time.
One side pole jumped with center. - result nothing idle speed remained the same (high).
Other side pole jumped with center. - result- idle decreased down to pre tps plug removal. (750rpm +-)

Removed TB - re-cleaned with tb cleaner, cleaned idle adjust knob.Reset screw on throttle shaft flange just touching with throttle closed plus 1/2 turn. Set idle adjust knob completely closed.

Reset the TPS so that it just clicked as throttle shaft was being moved.
Replaced gasket, tb, re installed linkage so that throttle was closed when cable spool was at rest.

Replaced a marginal small vacuum hose to charcoal canister.

Grounded red/white idle test wire (next to battery) started car result- -idle had decreased to about 600rpms.
Checked timing-result-12 degrees
with idle test wire grounded adjusted idle knob on throttle body to set idle at 725 + - rpms.
checked timing- results- 12 degrees
Removed idle test wire ground and idle returned to 750 rpms + or -.
checked timing- results- 12 degrees

Problems
The idle is still moving around, about 25 rpms on either side.
Still can not figure out why the timing failed the test. The timing has not been at TDC since I have been checking it.
Could the ECT sensor be causing the idle fluctuations?

I rechecked the emission test and there was nothing shown at idle.

Thanks for the help
Brad

You've sort of checked everything separately, but it sounds like you have not checked it as a complete unit. First, the two pins the you jumpered on the TPS that made a difference are the ones for idle mode. If the idle switch is (A) good (B) properly adjusted (C) throttle linkage is not holding it off idle, then simply watching the timing marks while connecting/disconnecting the TPS plug will show a change. If it passes that test... all that other checking, adjusting, testing is not needed. By not checking as a system, you have not eliminated A or C.

Sort of like getting out a voltmeter, ohmeter and whatever else to indivially test light bulbs, switches and wiring. If you turn the light switch on and the bulbs light, there's no need to check parts of the system.

If you confirm that the injection system is in idle mode (TPS circuit closed) and the timing is still 12 deg (assuming that you have not touched it since the e-test), then the test centre's pickup probe must have been misplaced.

I guess I didn't mention it, but there was a change in timing. :-) I went thru and checked everything at once, so I could be sure nothing else was adding to the problem. Any reason why the idle is still wandering around a bit? If the tps is working correctly, the iac is working, the throttle and linkage is adjusted, the TB is clean, the flame trap is clean, the hoses are good, new plugs, cap & distributor, new O2 sensor, what else could it be? I was looking thru my Haynes and it said a defective coolant temp sensor could cause irregular idle. What do you guys think?

Do you mean that there was a change in the timing when you unplugged the TPS connector? I do this with the engine running to avoid adding variables. If that is the case, then you have confirmed the operation of the TPS and can be confident that the reading you get (with the TPS properly connected) is correct.

You replaced a lot of parts that were very likely to have cause your emission test failure. Focus on that... not an idle speed that varies 25 RPM.

The answer is no on the temp sensor. Even if the circuit was intermittently opening or going out of range, its primary effect is on mixture, not speed. The ECU is programmed to not react to rapid changes in that input because normal operation does not occur that way. Those are exactly the type of malfunctions that would set a check-engine light if your system had it. Unless your car fails again for high CO (with normal other numbers), don't even consider the temp sensor as a possible problem.

Thanks again for good advice. I am going to see if it will pass on Monday. Will report back.
Brad

Let me first thank all of you for the suggestions and help. I took the car out for a drive yesterday to burn off any glue. Before I left I added some Redline fuel injector cleaner. I drove it for about an hour. Now currently the idle on the vehicle seems to have smoothed out according to the stock tach. The car ran pretty good. Went up the Cajon Pass @ 65-70 with no problem.

How accurate is the stock tach? The idle seems to be a little high according to it, 1000rpms. When I first start it it goes to about 1200 then settles at 1000. Yesterday I had my Sears tach hooked up and it showed the idle at 720-760 with the timing @12 degrees.

Here are the total test results from the pretest


HC
15mph max 116 meas 152
25mph max 92 meas 137

CO
15mph max .74 meas .76
25mph max .62 meas .71

NO
15mph max 791 meas 2537
25mph max 730 meas 2641

The report also said that my timing failed (Results Ignition timing TDC)
The timing I do not understand because it is supposed to be set at 12 degrees and thats what it shows on the timing belt case with the light. There is only 10 and 20 degrees marked on the case. Would the car even run at TDC? Could the belt have slipped and cause this?

I didn't get a chance to mess with the TPS yesterday and won't be able to check it till this afternoon. I will post back later after checking it.

I am assuming that you have only had one test done so far and that it did not fail a second test.

First; drive until that fuel with the injector cleaner is mostly gone, then fill the tank to dilute it. You've added a new variable by dumping stuff in the fuel.

I do not see result of any idle test. Cars are usually tested there too.

That 12 degree setting is only valid if the idle switch is closed. If the TPS does not close or is not properly connected, the timing would be advanced by anywhere from 8 to 15 degrees at approximate idle speed. The IAC would also be unpredictable. In other words; if the idle switch signal was missing, an actual static timing of TDC would result in a reading of 8-15 BTDC with the light. Engine power, gas mileage, and NOx would all go down slightly. In fact, all those vacuum delays used in early '70s cars was to lower NOx.

After confirming the idle switch setting (you would see a definite jump in timing if you unplugged a properly functioning one), recheck the ignition timing. Then ground the Red/Wht wire of the two pin connect near the w/s washer bottle and adjust the idle for 725 with lights and A/C off using your shop tach. The dash gauge is comparatively crude, especially at low speeds. Remove ground wire from test connector.

Thanks. I was planning on running the entire tank of gas out and refilling before the next test. Between my wife using the car and my work, I have not been able to work on it again. I am pretty sure the TPS is set correctly. It was one of the first things I checked. I will check it again tomorrow morn just to be sure. I will also check the timing with the TPS unplugged and see what happens. Will start early and post back as soon as possible.
Thanks again
Brad

You didn't state why it failed. Here in NY its easy to fail on NOX.. One issue I had on my 87 740 was the intake manifold gasket had deteriorated and the ground cable had become corroded. Once both were (separately) relaced, the engine was smooth, quiet and in well below NYC standards on NOX, CO etc.
Just a thought...

Dr. Chip
Westchester County NY
98 V90.

Update. The car after waming up is now idling between 720-760 rpms, I checked the timing and it seems to be @12 degrees. I re-checked all the vacuum hoses. I think I will get a can of injector cleaner and take it out for a long drive. Will report back. Just had a thought about the fuel filter, would that cause the uneven idle or could it possibly be the fuel pressure reg? I will be back on here in a couple of hours.
Thanks Brad

A clogged fuel filter would show up as starvation under hard acceleration, not an idle or starting problem.

A partly clogged injector could cause a poor spray pattern and would appear is slightly high HC. It would only occur if it was severe enough for a lean misfire.

If the guy that replaced the cat was truthful, and the old one was empty... somebody had to manually knock out the ceramic. It didn't happen by itself. Only a few minutes burn-in time is needed to clear the new exhaust parts of assembly chemicals. You don't need to drive it far.

Emission limits vary by vehicle weight and jurisdiction. The results are only slightly helpful without knowing the limits for each test. Post the limits with the test results if the car fails again.

My 87 240 cleared out its own cat. The ceramic substrate was GONE after a few months of rattling.

And of course, it failed miserably on NOx. Other readings all within spec.

I agree that the cat ought to work almost from the second it's put in.

I think this car's gonna have to get under 1700 or 1800ppm to qualify in the eyes of the C A R B.

In the case of my 87, I had a complete exhaust from an 89 parts car, and I bolted it on and drove it right to the emissions test station. It passed on the first try, nearly perfect numbers.

Right now I have a neighbor's 88 240 to get through the test. Same deal, good CO and HC, high Nox. (>2200). So I'm looking around for another good exhaust system...

Interesting note, in CT, if a car fails 4 times, it goes onto some sort of double-secret-probabation blacklist not mentioned on the DMV website. You don't even get a free retest, even if you just bought it. Totally bogus law and completely anti-incentive for me to fix the thing in a timely manner. Here, have some more acid rain. Don't like it? Fine, I'll drive my 70 Bus... enjoy my uncatalyzed output!
--
Rob Bareiss, New London CT ::: Today I'm driving: 1990 745GL, M47, 273K, Hydras, Urethane bushings, Teal & Tan ::: My wife's got my 92 down in VA Beach!

I should have said that it didn't happen by itself without consequences. Often when the ceramic breaks apart, the pieces lodge in the first tube of the muffler and eventually clog it. The cat failure is discovered when the clogged exhaust is replaced for no-power on acceleration. Because of muffler design, those bits-o-cat do not go out the tailpipe.

The speed adjustment knob is simply an air bypass. Its in and out ports are in parallel with the IAC. The unsteady and high idle, would suggest the throttle is not closing all the way (probably linkage adjustment) and preventing the TPS idle switch from closing. You should hear a distinct "click" at the TB as you get to or come off of idle position.

You are unclear about whether your failure was after all those parts were replaced, and whether the failure was at idle or under load. If it fails again, post ALL the numbers for gasses, including the limits for each mode.

Just getting ready to leave. The idle has changed again. It is now about 800. I will check the tps again. It was set so that as soon as the trottle was opened just a hair it would click. The smog test failed before all the work was done. Sorry I wasn't clear on that point. The vehicle was a gross polluter and failed on both the 15 and 25 mph tests. The cat was completely gone. The installer said there was nothing left inside the can.

Here were the test results

HC CO NO

15mph 152 .76 2537


25mph 137 .62 2641

The cat installer said I needed to drive the car for a while to burn off the adhesives in the cat before it would pass the test. So I was going to go do that and possibly run some techtron thru it. What do you think?

Thanks Brad

Remember, you have to shut down (ground out) the IAC motor in order for the knob on the throttle body to have any effect. You sound as though you know this, but just checking...

You know that if that car is due up for it's regular biennial inspection, it is currently registered in your name, it was test-only directed, it has no missing modified or disconnected equipment, and it failed then you are eligible for up to $500.00 worth of repairs from the state. There is a hundred dollar co-pay but if you have spent all the money that you said have on emissions related stuff in an attempt to pass and inspection then you need only submit that stuff with the forms and your co-pay will be met. I do not know if this applies to you but if it does it is free money.

Mark

Those exemptions are allowed and based on the idea that the car is near the end of its life and will be off the road within the next year or so. Many states do not allow future exemptions and some limit the number of times a car may fail emission tests.

With the cost of fuel these days, having the engine pass indicates that economy will probably be the highest it could be based on the engine's mechanical conditions. Worn spark plugs and lazy O2 sensors account for a huge percentage of failures (one for HC, the other for NOx). Each will cost gas mileage.