1990 Lumina Hesitation / "putt" @ idle

This is a problem that will require a rather lengthy post because it's a long story! :) It begins with a 1990 Lumina A/T 3.1 multi port. The initial problem was rough unstable idle with a hesitation at the throttle "crack point" (just above 0%) throttle position. I found a bad fuel pressure regulator which was sucking gasoline from the bad regulator diaphragm into the throttle body. This was very straight forward and a no-brainer. I replaced the regulator which cured the problems. A couple of weeks later the hesitation returned along with a "putt-putt" sound from the exhaust, not a direct engine misfire just an irregular "putt-putt". I checked the new regulator with a vaccuum tester and found no problem, I then began to search for vaccuum leaks and found none. I ran a fuel pressure test and found my fuel pressure to be 35psi at idle and 36 psi @ 2000rpm. I hooked up a snap-on MT2500 diagnostic tester and ran a scan. The results indicated a lean condition; Oxygen sensor readings were stuck at 4mvs and exhaust oxygen was always lean. I began my search for the cause which entailed the following; EGR inspection and test (ok) MAP sensor vaccuum test,signal test,and voltage test (ok) PCV system test, replaced PCV valve and all hoses, removed the throttle body and dis-assembled, cleaned with submersible parts cleaner, and replaced gaskets, checked IAC ports for carbon and cleaned. Removed the air intake plenum and cleaned and inspected for cracks and carbon issues, removed fuel rail and injectors inspected and cleaned fuel rail, tested all injectors for proper resistance all were between 15-20 ohms, ran carb and choke cleaner through each one and blew out with compressed air. I replace the oxygen sensor. I re-assembled the thing and when I started it everything was back to normal. My hesitation was gone and the on-board computer was happy, my air fuel ratio was 14:7 and my lean condition was gone, I thought I had conquered!! I drove the car for about four days and (oh no!) the hesitation is back, along with the "putt-putt" from the exhaust. I ran an engine scan and everything looks in order, it has not set any codes and all parameters seem happy, my air fuel ratio is right on at 14:7
I have checked the spark advance and it seems to be responding normally and I have re-checked fuel pressure and it is running at 36psi, my TPS is responding from.64v @ 0% to .96v @ 10% which seems normal to me. I have a feeling this could be mechanical or possibly an ignition problem. I am hoping someone out there maybe has expierienced the same symptoms and can give me some help.

Have you changed fuel filter yet?
According to the GM Lumina service manual fuel pressure should be between 40.5 & 47.5 PSI on the 1st generation 3.1 MPFI engines (the one I refer to is one borrowed from the Chev dealer shop). Resistance @ injectors should be 12 +/- .28.

Yes I have changed the filter, the pump is about 2 years old. As far as the pressure is concerned, you raise a point. I have two manuals (Chilton and OTC) each manual has a different range in regards to fuel pressure. The OTC does say 41-47psi, Chilton say's 24-40psi. My OTC manual does say the injector resistance should be be 12-12.6 ohms, however this resistance reading is at 60 degrees C. (140 degrees F) I tested my injectors at room temp and all were the same, which makes me believe they are ok (in regards to ohmic value) The thing is, this thing runs great, no missing, loss of power, just good performance. Wouldn't you think a bad pump would show its ugly self under power??

In your posts, I haven't seen any mention of tune-up history (ie plugs, wires) and also, what brand oxygen sensor did you install? GM cars run like dog-doo on Bosch.

For clarification, the correct fuel pressure spec I show matches what bearwulf listed, 41-47psi key on/engine off, and a 3-7psi drop (38-40 minimum, with 40-44 psi preferred), car running with FPR vacuum attached. Your fuel pressure seems a little low, but could be considered within spec allowing for a 5-10% margin of error on your guage.

The spark plugs have about 20,000 on them. I removed the plugs this weekend to run a compression test. They looked normal, with a bit of wear. I adjusted the gap and re-installed them. The plug wires were installed with the spark plugs. (I had to replace the head gaskets at this time, I also replaced 2 coil packs, anti-freeze had infiltrated them) My compression ran between 170-180 psi on all cylinders. I can't tell you what brand the O2 sensor is. I bought it at a local parts store, however, I have observed it's function with my scan tool and it seems to work flawlessly. The block learn sets around 100 the integrator is at 128 the air fuel ratio is 14:7 and it ranges from lean to rich like a normal sensor would. From what I have read these parameters are pretty normal. I have access to an ocilliscope and I have considered hooking it up, but I don't understand why it runs out well. I would expect ignition breakdown under power. What is your opinion?

I've owned a couple cars with these engines, and one always had a little putt-putt at idle like you describe. Since the car ran flawlessly otherwise, with good power and mileage, I never bothered to try and fix it. If this car has the factory dual exhaust, it may be normal behavior.

One last thing, have you done vacuum test and/or exhaust back pressure test to rule out a bad/partially clogged converter?

I have had a vaccuum gauge on the engine, (I was checking the vacuum at the MAP sensor), at idle it was around 25 inches. The hesitation at throttle crack is the real problem, I just figure the "Putt" out the exhaust is related to the hesitation. The thing will die every once in a at throttle crack (when pulling away from a stop) This is the annoying part, especially when my daughter is driving!
Thanks for all your help!

Your vacuum reading is a little on the high side, but not out of spec. The only thing I can think of that would cause what you're describing is either a questionable IAC motor that isn't adjusting quick enough on throttle tip-in, or the throttle position sensor has a small flat spot that you're not picking up while testing.

One last thing, is the air filter new/relatively clean?

It was new last oil change.

This is a problem that will require a rather lengthy post because it's a long story! :) It begins with a 1990 Lumina A/T 3.1 multi port. The initial problem was rough unstable idle with a hesitation at the throttle "crack point" (just above 0%) throttle position. I found a bad fuel pressure regulator which was sucking gasoline from the bad regulator diaphragm into the throttle body. This was very straight forward and a no-brainer. I replaced the regulator which cured the problems. A couple of weeks later the hesitation returned along with a "putt-putt" sound from the exhaust, not a direct engine misfire just an irregular "putt-putt". I checked the new regulator with a vaccuum tester and found no problem, I then began to search for vaccuum leaks and found none. I ran a fuel pressure test and found my fuel pressure to be 35psi at idle and 36 psi @ 2000rpm. I hooked up a snap-on MT2500 diagnostic tester and ran a scan. The results indicated a lean condition; Oxygen sensor readings were stuck at 4mvs and exhaust oxygen was always lean. I began my search for the cause which entailed the following; EGR inspection and test (ok) MAP sensor vaccuum test,signal test,and voltage test (ok) PCV system test, replaced PCV valve and all hoses, removed the throttle body and dis-assembled, cleaned with submersible parts cleaner, and replaced gaskets, checked IAC ports for carbon and cleaned. Removed the air intake plenum and cleaned and inspected for cracks and carbon issues, removed fuel rail and injectors inspected and cleaned fuel rail, tested all injectors for proper resistance all were between 15-20 ohms, ran carb and choke cleaner through each one and blew out with compressed air. I replace the oxygen sensor. I re-assembled the thing and when I started it everything was back to normal. My hesitation was gone and the on-board computer was happy, my air fuel ratio was 14:7 and my lean condition was gone, I thought I had conquered!! I drove the car for about four days and (oh no!) the hesitation is back, along with the "putt-putt" from the exhaust. I ran an engine scan and everything looks in order, it has not set any codes and all parameters seem happy, my air fuel ratio is right on at 14:7
I have checked the spark advance and it seems to be responding normally and I have re-checked fuel pressure and it is running at 36psi, my TPS is responding from.64v @ 0% to .96v @ 10% which seems normal to me. I have a feeling this could be mechanical or possibly an ignition problem. I am hoping someone out there maybe has expierienced the same symptoms and can give me some help.

For many years I had intermittent idle surge problems with my 1990Lumina Euro 3.1. After much work on it myself, I had several service stations work on it. Fuel pump and relay, regulator, computer, and injectors were replaced. The throttle body was cleaned. The throttle position sensor, and MAP sensor were replaced. The computer did not code out. The oxygen sensor also was changed, and I did find that using an AC Delco sensor got better results than any other brands, but surging still occurred at idle. The car ran O.K. when cold, got lumpy surging idle when it warmed up. That was the key, the idle became lumpy when the ECM went into closed loop, at aboput 140 degrees F. Continuity from the single wire O2 sensor to the ECM read good, which delayed repair. I finally cut the wire to the O2 sensor, purple, and having the factory repair manual showed the ECM pinout , ran a new one along the fire wall, inside a split loom sleeve, using crimp butt connectors, and sealing the ends with silicone sealant for waterproofing. Problem solved. Apparently, over many years, oil leaking on the wire had broken down the insulation, and the very weak signal from the O2 sensor was leaking off to chassis ground. Valve cover oil leaks had also wreaked havoc with the Distributorless Ignition System, mounted beneath the front valve cover, and the Crank Position Sensor, on the rear of the engine, beneath the rear valve cover. The silicone rubber gaskets on the coil packs were not sufficient to keep the oil from shorting the primary circuit. These engines also leak oil from the plug where the distributor used to be, repaired by replacing the O ring, and from the top of the front cover, which so far has responded to a bottle of CD2 engine oil stop leak, but may need replacement or retorqueing of the bolts. My experience has been that unlike older pre-computer vehicles, oil leaks and elctronics don't work well together. Those leaks require a lot of labor to fix. but left unrepaired, they will continue to take out electronic components, or interfere with the sensor signals arriving at the ECM at their intended level and pulse duration.

2 out of 4 Luminas that I have '90-'93 w/3.1s, have had the "putt"@ idle problem. The '93 had that from the day it was new (driven from dealership lot), which I learned from the person that I bought it from who was the original purchaser. It died about 1.5 years back @ approx. 380,000 miles.
The '91 that I drive now also has the "putt" and is still going strong @ about 250,000 miles.
I wouldn't worry too much about it.

2 out of 4 Luminas that I have '90-'93 w/3.1s, have had the "putt"@ idle problem. The '93 had that from the day it was new (driven from dealership lot), which I learned from the person that I bought it from who was the original purchaser. It died about 1.5 years back @ approx. 380,000 miles.
The '91 that I drive now also has the "putt" and is still going strong @ about 250,000 miles.
I wouldn't worry too much about it.
The surging idle and hesitation is gone since I replaced the wire from the O2 sensor to the ECM. That wire, tested good with a digital volt meter, good continuity, no continuity to chassis ground, apparently had broken down compromised insulation from being soaked in oil for many years. The very weak milli volt level signal from the O2 sensor was not arriving with sufficient strength at the ECM. Cutting the wire at both ends, and runnigng a new one in wire loom along the firewall, wire tied to the heater pipe, cured the problem, and it is now not in a location subject to oil leaks. The engine runs better at all RPMs now that the ECM can accurately adjust the fuel mixture once it goes in to closed loop, at about 140 degrees F. I thought that I would share that fix with other Lumina owners who are experiencing the same frustrating problem.

As Bearwulf said, your values are off for both fuel pressure and injector resistance. You either have an inaccurate fuel guage and voltmeter or there's a problem in those areas. The injector resistance could be the problem and/or the fuel pressure.

Did you clean the throttle body? Does the problem show up when the engine's warmed and in closed loop, and do you have oil getting on wiring? It might be the problem that dannym discovered with the O2 sensor. You might also want to check the ECM wire to the Coil pack module to make sure you don't have an oil/antifreeze soaked problem there (Iremeber you said antifreeze had gotten in there), and maybe check the ignition timing at tip-in when you get this hesistation. An exhaust leak at the exhaust manifold could cause the O2 sensor to read lean, but I doubt that'd cuse your hesistation.

The injectors on thse cars are a common problem area. Anyway, you need to get to the bottom of your low fuel pressure, that could be the problem there.

This is a problem that will require a rather lengthy post because it's a long story! :) It begins with a 1990 Lumina A/T 3.1 multi port. The initial problem was rough unstable idle with a hesitation at the throttle "crack point" (just above 0%) throttle position. I found a bad fuel pressure regulator which was sucking gasoline from the bad regulator diaphragm into the throttle body. This was very straight forward and a no-brainer. I replaced the regulator which cured the problems. A couple of weeks later the hesitation returned along with a "putt-putt" sound from the exhaust, not a direct engine misfire just an irregular "putt-putt". I checked the new regulator with a vaccuum tester and found no problem, I then began to search for vaccuum leaks and found none. I ran a fuel pressure test and found my fuel pressure to be 35psi at idle and 36 psi @ 2000rpm. I hooked up a snap-on MT2500 diagnostic tester and ran a scan. The results indicated a lean condition; Oxygen sensor readings were stuck at 4mvs and exhaust oxygen was always lean. I began my search for the cause which entailed the following; EGR inspection and test (ok) MAP sensor vaccuum test,signal test,and voltage test (ok) PCV system test, replaced PCV valve and all hoses, removed the throttle body and dis-assembled, cleaned with submersible parts cleaner, and replaced gaskets, checked IAC ports for carbon and cleaned. Removed the air intake plenum and cleaned and inspected for cracks and carbon issues, removed fuel rail and injectors inspected and cleaned fuel rail, tested all injectors for proper resistance all were between 15-20 ohms, ran carb and choke cleaner through each one and blew out with compressed air. I replace the oxygen sensor. I re-assembled the thing and when I started it everything was back to normal. My hesitation was gone and the on-board computer was happy, my air fuel ratio was 14:7 and my lean condition was gone, I thought I had conquered!! I drove the car for about four days and (oh no!) the hesitation is back, along with the "putt-putt" from the exhaust. I ran an engine scan and everything looks in order, it has not set any codes and all parameters seem happy, my air fuel ratio is right on at 14:7
I have checked the spark advance and it seems to be responding normally and I have re-checked fuel pressure and it is running at 36psi, my TPS is responding from.64v @ 0% to .96v @ 10% which seems normal to me. I have a feeling this could be mechanical or possibly an ignition problem. I am hoping someone out there maybe has expierienced the same symptoms and can give me some help.

Hello
I have "EXACTLY" the same problem with my Lumina. Just changed the O2 Sensor wire as it says further on in this thread. I also changed the fuel pressure regulator, cleaned and checked injectors etc etc. Hunts at idle if idle is 650 rpm. If I raise idle to 1000, sort of smooths out. Runs perfect on the road. Stalls once in a while when stopping from high speed. Tried dirrerent IACs, MAP sensors, No Vacuum leaks found. Changed Catalylic Converter. But could a small exhaust leak at the front pipe cause this?
No codes stored from scan, but Block Learn is around 100, a Rich condition?
Any ideas?

Do the basics first. Computer controlled engines run very lean, and require proper and consistent exhaust back pressure to run properly. The IAV can only compensate for a limited range, so the engine will surge while the computer tries unsuccessfully to stabilize the idle. Check for intake manifold leaks by spraying carb cleaner around the gasket. If the engine speeds up there is a leak. I had a crack in the pipe just before the catalytic converter. Put the car up on ramps, and with the engine running in Neutral, look and listen, and feel for exhaust leaks under the car. I replaced the entire exhaust system, but in retrospect should have had it welded. OEM is much heavier, and the OEM Cat works better than aftermarket. Also look for intake leaks, worn, loose fitting rubber hoses to the fuel pressure regulator and MAP sensor. These engines also develop coolant leaks from the intake manifold. Use the powder stop leak tablets for a fix, they are factory installed on many GM engines. Repair oil leaks as described in my other posts. Clean the coil packs of oil that gets into primary connections, and replace the rubber seals. Clean the crank position sensor plug. It feeds the ignition module, tells it when to fire. Check ignition wires for breakdown (burn marks), have a mechanic scope the engine.. I installed Autolites that failed in less than a year! My mechanic found that problem. Use AC Delco if available. Fuel pressure gauges of the home mechanic variety aren't very accurate. I read about 36 PSI, my mechanic told me he read 41PSI, both at idle. I had already replaced the pressure regulator and filter. I had the pump replaced only because the car was very hard to start in hot weather after the engine had run for a while. If it runs OK except at idle, the fuel pressure is probably OK. The pump must supply much more volume at high RPM's than at idle. If the idle is fairly smooth when the engine is cold, and then gets lumpy at about 140 degrees, that is when the computer stops relying on built in algorithms to set the mixture, and goes into closed loop, relying on the O2 sensor feed back to set the mixture. That indicates a lean condition. Fuel injector resistance readings when cold aren't very reliable, because when they get hot, the windings can short. I was told by another mechanic that my injectors were bad. I replaced all of them with new aftermarket BWD from Advance Auto, about $60 each. The idle did not improve, but the engine has more power. The only things that improved the lumpy surging idle were replacing the O2 sensor wire, and fixing the exhaust leak. The idle was smooth at 500 RPM in gear when I bought the car used with about 70,000 miles, and it is smooth again. I had raised the idle to prevent stalling, but have now reduced it to between 500-600 RPM in Drive.. Most of the newer Chevy's idle around 600 RPM. My Lumina was about 500 when I got it. That is a low idle, just like my old 1967 283 V8 was. It is good for fuel economy and engine braking, but leaves little room for error now that the pollution controlled engines run very lean. Again, do the basics first. Look for exhaust and intake manifold leaks taht will cause a lean condition at idle.

Well, I looked at the exhaust system again, tightened a few clamps. As I moved to the rear of the car I heard a vacuum leak! Found a broken rubber hose to the canister. The way it was positioned, I did have to drop the tank a bit to remove the broken hose from the metal line, but now with new lines, it idles as smooth as it did 10 years ago.

I was told to look for a vacuum leak a while back but did not expect to find it at the rear of the car. But, I did refresh and replace a lot of things along the way and learned a bit, too.

Dear goodo,

That was a good find. I also forgot that the vacuum canister is located there. Thanks for the update, and wishing you good motoring!

dannym

The reply from Dannym was very detailed and seems to be right on for this problem.
The symptom as he described it:
..." the idle is fairly smooth when the engine is cold, and then gets lumpy at about 140 degrees, that is when the computer stops relying on built in algorithms to set the mixture, and goes into closed loop, relying on the O2 sensor feed back to set the mixture. That indicates a lean condition. "
Seems to be exactly what I am now experiencing. Since I already have replaced the O2 sensor wire with a shorter, direct wire as he suggested, I now have to fix the exhaust leak
between the exhaust manifold and the front pipe, forward of the catalytic converter.
I'm sure that the gasket or donut that is in there is pretty much shot since every time you change spark plugs and tilt the engine/transaxle, this part must get stressed.
Just to summarize, this is after replacing: Spark Plugs, Wires, IAC, MAP Sensor, Fuel Filter, Fuel Pressure Regulator, and also after repairing numerous vacuum leaks including one near the fuel tank. I also cleaned the Injectors twice, once with a Pressurized tool running Seafoam, and again when I had the Fuel Rail out to replace the regulator.
There really can't be anything else, or can there?
Dannym, once again, thanks for your advice. When I fix the exhaust leak, I'll conclude this long long story. 278,000 miles long.......