ED HEISER

I have an 88 coupe A4 with 170 k miles. It has started running poorly after it warms up completely (15 minutes or so). The weather has suddenly gotten very warm, I don't know if the cold weather was masking this problem or it is something new. It starts up and has full power, then it starts running on 6 or 7 cylinders when hot is how it feels. I checked the codes, nothing but 12,12,12 which means no codes. I changed the plugs, wires, cap and rotor. The cap had been assembled wrong by a previous owner or his mechanic. The carbon brush was on top of the rubber washer, and the rotor ate up the plastic on the inside of the cap to get to the brush. How it ran like this so well is a mystery to me. I was thinking of replacing the coil and module, but I hate to throw good money away. Do you think the cap/rotor problem could have damaged the coil or module? What else could make it miss when hot, and yet run perfect when cold, yet throw no codes? I would appreciate any advise you can give.

RRT vette

O2 sensor

ED HEISER

Could you elaborate?

VenkmanP

When a car runs fine when cold and then badly when warmed up, it's a problem with the O2 sensors.

ED HEISER

OK, thanks. I just got back from Aotozone, they had one for $22.50. If this solves my problem I will be a happy fella. BTW, where does this puppy go? I left my manual at work.

rick lambert

Well, if that doesn't cure it, pull the ignition module and have Autozone check it-they run them through 10 cycles, get them hot to see if their failing.

RRT vette

The O2 sensor is a open/closed circuit and is open when the exhaust temp is under 600*f or engine temp approx 140*f. In open loop the o2 sensor circit is open circuit (no voltage) and not contrubiting to the idle or drivability. The ecm uses a assortment of other sensors to operate idle and drivability in open loop. When the temp criteria as been met the ecm reconizes this an goes in to closed loop and uses the O2 sensor mainly for fuel command. High voltage when rich and low voltage when lean. Now, when the ecm has seen voltage out of is range it should throw a code 44-lean, 45-rich, or 13 open circuit. Not alway will these codes show up if the o2 sensor has silicon contamination or faulty. I would think the plugs, wires, cap, and rotor could show a drivability when the car has reached operating temps (this can be tested as wellas plug wire resistance) . Does your exhaust smell rich when cold or hot? Have you checked the plugs after they were changed (color and smell)? I'm not telling you to get a new O2 sensor but to troubleshoot the O2 sensor because I would think this could be the problem. (Example) I have a 95 vette that ran rough when hot. No code showed up and it acted like it was running off of 7 cylinders. I had a scan done and it showed the left front sensor was not functioning correctly. I changed it and it fixed the problem. If you have a FSM, study it or maybe even have a scan done. IMO

RRT vette

O2 sensor on 88 goes in the left side exhaust just under the brake booster. If you plan to replace before testing, run engine a little to heat up exhaust (makes it easier but hotter), disconnect the wire, pull old sensor out and install new sensor (antiseize should already be on threads) and tighten to specs or snug. Connect wire and make sure nothing is touching exhaust. Cranker up and pray. The o2 sensor will smoke for a minute or two when getting hot.

ED HEISER

Thanks for the explaination, I will change the O2 sensor in the morning when it's cold. If this does not solve the problem I will have it scanned, but it certainly sounds like the problem. It does smell rich when hot after I pull into the garage. The plugs look light tan in color. I just changed the cap,rotor, plugs and wires so that's not the problem. I got a tankfull of gas just before this happened at a station I never used before, maybe it contaminated the O2 sensor, or was the last straw. I found it on rear of the exhaust manifold on the drivers side, looks like it's the original to me.

ElisTwoCents

Hmmm keep us posted then, I too have the same issue. Runs great when cold and bad once warmed up or drive for 20 minutes or so. Sputtering and such.

Bubba91

ME TOO!!!!

Morley

You can also try the coolant temp sensor. I posted a chart here once on the ohm readings it should have for various temps.

RRT vette

Good point as well!

mudslide

Sounds just like what im going through with my 1990 it runs great under trottle and cold but when warmed up it idles like crap and if you try to hold it steady at 40mph 4th gear 1800 rpm it bucks back and forth. let me know what happens from here--so far new cap rotor plugs cleaned TB and iac ohmed the wires --whats next??

rick lambert

I found it easy to remove the 02 sensor while the engine was still cold.

Mike_88Z51

This is so absolutely wrong.

The O2 sensor is NOT a open/closed circuit unless it is defective. The O2 sensor supplies a VARYING voltage that goes from 0.100mv to 0.900mv rapidly when working properly. An O2 sensor in the older L-98 cars does not have the ability to be self heating so when the tip temperature drops below 600*F it does not supply an accurate voltage to the ECM and the ECM is programmed to go back into open loop, but many of the newer cars LT-1s use a heated O2 sensor which allows the ECM to stay in closed loop during idle and lower coolant temps. Additionally, the O2 sensor tip may come up to temp when first started, but the ECM is programmed with a timer that prevents it from going into closed loop for a minimum amount of time. This period varies throughout the years, but is generally in the 2 minute range.


True often enough, but Not Always. It is possible that the injectors are being affected by engine heat and start acting up in a way that they don't when cold. A good O2 sensor and ECM cannot see or make adjustments to resolve a problem in an L-98 like ED's with a bad injector(s) on the passenger side of the engine. There is only one O2 sensor in his 88 and it is reading and reporting exhaust mix based upon the driver side injector bank operation. Also, the 85 - 88 L-98 had a 9th injector referred to by GM as the Cold Start Valve or CSV. The CSV is controlled by a seperate thermal time switch that has no interaction with the ECM. If the CSV is working during cold start and not turning off when it should or leaking when hot, it can affect cold starts in a positive manner and warm starts/running in a negative manner.


Most of this is basically correct and good advice.

I do agree with RRT vette and Vis Croceus that a car that runs good cold, but runs poorly when warmed up should cause you to suspect and check the O2 sensor and circuit as a first step. It could be the sensor is bad, or it could be a sensor circuit connection problem. As Morley already mentioned, it could also be the coolant temp sensor. The 88 uses 2 coolant temp sensors. One for the digital dash guage and one for the ECM to read coolant temps. Your dash coolant temp reading might be fine and the one used by the ECM defective. The ECM uses the coolant temp in conjunction with the O2 sensor feeedback and MAF airflow readings to decide what the ON time for the injectors should be. If the ECM thinks you are cold and you are, it will run fine. When you warm up, it needs to lean the fuel mix a touch and it won't do a good of that if it still thinks the car is cold when it is warm. In the 88 the coolant temp sensor is at approx 100k Ohms (High resistance) when reading cold (40*F) and 70 Ohms at 266*F. A defective but not completely bad coolant temp sensor could easily be reporting an overly high resistance and throwing off the ECM.

First thing I'd get a scan check of the O2 sensor. If the car has 170k miles and appears to have an original O2 sensor you are long past due for a new one, I'd just replace it as they do age and wear out. You want to disconnect the battery voltage for 10 seconds to clear out codes in the ECM and to clear old BLM (Block Learn Multiplier) values so the ECM "re-learns" the operating parameters using the new O2 sensor. If that doesn't completely solve the problem, I'd make a resistance check of all 9 injectors when hot and cold. In the 88 they should all be above 12 Ohms (usually around 16/17 Ohms) and all within an Ohm of each other. If they don't test to spec, get them cleaned/rebuilt or replace them. If they are ok then I'd check the ECM coolant temp sensor resistance for accuracy.

RRT vette

QUOTE from 86 FSM ..."The sensor is like an open circuit and produces no voltage when it is below about 360* (600*f). An open sensor circuit or cold sensor causes open loop operation."
True Mike, You got me, the O2 sensor is not exactly an open/closed and in fact does get supplied .1v-.9v by the ECM when working properly. Open loop/Closed loop...misinterpretation on my part.

rick lambert

That coolant temp sensor was a good call. Had a friend that chased an itermittent that did not throw a code, but it did turn out to be the CTS.
I'm just learning scanning, but from what I've seen so far, scanning and understanding the 02 should be fun-fluctuates so damn much, but with help I'll figure it out. Remember-you can have the ignition module checked for free at autozone.

Mike_88Z51

Well, you tried to describe it like the FSM, so I apologize if for jumping on your answer. I see much better why you stated your point the way you did. The FSM while obviously accurate, dosen't make it real clear and can easily be misinterpreted. I was just trying to keep anyone new from thinking that the O2 sensor was sort of fixed circuit with on/off states that put out a steady state type of voltage.

In a good properly working O2 sensor, when the tip temp reaches 360*C (600*F), its output voltage jumps rapidly between the high and low values. As they age, O2 sensors lose accuracy and the transitions get slower with the voltage output peaks becoming lower. A "working" O2 sensor can appear to be fine, but due to age it may be losing accuracy. Even though no codes are set and it is making voltage transitions, it may not be as accurate over time. This can lead the ECM to make less than optimal mixture decisions based on the less accurate feedback from the O2 sensor that appears to be working correctly. I'm now of the opinion that far too many of us wait for the O2 sensor to completely fail and set a code when the sensor should be replaced at regular intervals like a fuel filter. You don't wait for the filter to clog completely before replacing it. Same for O2 sensors, probably should just replace after 50k miles unles you have a scan showing it to be in good and accurate working order.

The O2 sensor is basically a heat activated chemical analog voltage generator. The ECM doesn't supply the 0.100mv-0.900mv voltage. That is what the O2 sensor supplies to the ECM based upon the difference between ambient air O2 and exhaust O2 differences. The ECM reads and compares the O2 sensor voltage against a base reference voltage of 0.450mv to define whether or not the mixture is lean or rich. Continuing averages above 450mv is an indication of a rich condition and below 450mv is a lean condition.

rick lambert

You know Mike, I subscribe 100% to the maintenance factor, but I think even going 50K miles on our high mileage C4s may be stretching it
without being able to scan it-I'm just in the learning curve with scanning-but I can certainly see the benefits. I suggested at one time for the relays also-got blasted for it, but hey, they can effect how the car performs-or doesn't, or even point a finger to something else-and you can't scan a relay! The 02 is what? $25-30. relays are $15. and Mobil1 is how much?