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SUVs Chevrolet Suburban & Tahoe Chevy Suburban Rough Idle and Stalling
95 messages, Last post on Nov 03, 2009 at 6:57 AM
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Replying to: alanrs (Jun 17, 2008 5:13 pm) Here I explain what happened to my "04 Tahoe 5.3L Flex Fuel engine very soon after I got the car. I have posted about my problems several times in this forum before but this all is very difficult to find I assume. Here are a couple of easy checks. 1. After only about 1200 miles my Tahoe started eating too much gas. This means 18 mpg went down to 13 mpg on highway driving. During the trouble shooting process, which I had to do myself while the dealer did nothing during warranty period, I first changed spark plugs. After that I get misfire trouble codes. As I had only changed the plugs this obviously had to be related to plug change. As a result it ended up being very simple problem. Some of the aluminum metal shields around spark plug booths that my Tahoe engine have were grounding spark to engine ground. For trouble shooting purposes I removed the metal grounding spring from the end of the shields, which allowed me to even see the problem, i.e. when it was dim lighting conditions I could see a spark between some of the shields and engine parts, mostly exhaust manifold. When spark plug wires get old they can crack and other vice loose isolation properties and start leaking spark to the shield. Because the shield is grounded to the engine with that spring in the end of it the spark has direct contact to ground. This will lead to misfire as the resistance for spark forming changes. This same problem easily happens when spark plugs are changed. When the plug wires are pulled from the plugs sometimes they are very tight. This can cause the plug connector inside the boot slide out. When this happens the boot does not go over the new spark plug deep enough leading to spark leak around the boot to shield and this was exactly what happened to me. Some of the wire boots did not go on plugs deep enough and spark leak was very bad. This can also happen with new wires as all of them do not have plug connector deep enough inside the boot. There is very easy way to trouble shoot for this, i.e. remove all aluminum boot shields. They are used only for heat shield purposes so you can take them out and drive a few miles and see if that fixes the problem. I actually took mine out and drove for a week without them. I could smell a little burning rubber but they did not catch fire or anything. Then I went and bought a set of high performance spark plug wires for $5 less than stock wire set from dealer cost, a set that does not require using boot shields and have been trouble free from this problem since. It was not the cause for low fuel mileage though. If your engine does not have the aluminum heat shields around plug wire boots then above does not apply to you. 2. This happened to me just 2 weeks ago. I was working on the car again for fuel mileage, which has been back to around 17 mpg highway but not 18. I decided to change O2 sensors, the first one in each bank. Went to Auto Zone and they had a Bosch sensor for $65.00. This sensor was of the universal type, i.e. It did not come with wire harness and I had to use the one from old sensor. Further more, the instructions in the package did not explain which wire is the signal so I tried to guess. As we all know Mr. Murphy does not take days off so my guess was wrong, i.e. it is the BLUE wire that is the signal, not white. But very interesting thing happened while I tried to run the engine with wrong O2 sensor wire as signal wire. I got probably all of the misfire codes the computer can give. And the engine was running VERY ROUGH! So, your problem could be with oxygen sensor too. It just takes a new sensor to try. There are ways to test a sensor but if you already have 80 000 or so miles on it it might be a good time to change O2 sensors anyway. Arrie |
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Arrie, I agree with your analysis -- however, alanrs seemingly has access to a Tech2 autoscanner. These scanners are what the dealers use and they cost around $8000.00. A bad O2 sensor would have already been indicated. Also, an O2 sensor is nothing more than a fuel delivery device. It measures the amount of oxygen in the exhaust and tells the computer to either richen or lean out the fuel delivery. I agree that with 80,000 miles, changing them is not a bad idea -- but I do not agree that they are causing this mis-fire condition. This engine has coil over plug wires meaning that they are very short; the reason for this was an attempt to reduce the resistance and arc travel time to the spark plug. It was stated that the wires are A/C Delco ( one of the best ) and they are new. Yes, those heat sheilds can be a problem, but pulling the plug and looking at the condition of that plug will tell you whether or not that particular plug is firing properly -- meaning -- burning all the fuel. It will also tell you whether or not that there is enough fuel getting into that cylinder and also, whether or not there is coolant leaking into the combustion chamber....Low fuel pressure seems to be the problem in this case--but we'll just have to wait and see----
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Replying to: pear69 (Jun 22, 2008 9:04 pm) Just trying to help... I forgot to mention that when I had the O2 sensor signal wire connected wrong I DID NOT get a code for O2 sensor...go figure...perhaps thing are not always what they should be. Removing heat shields around spark plug boots is so simple thing to do too that I would not hesitate trying that. And sometimes thing help that don't seem they should. There are times when something fixes the issue that should not. I would take the fix and think later why it helped.
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Replying to: arrie (Jun 23, 2008 2:38 pm) Today while driving I noticed that the misfire had magically disappeared. Sure enough, smooth idle, no "chuggle" when I lift foot off of brake pedal... Of course, I expect it to return, sooner than I'd like. Will pull code(s) before next drive. SES light has been coming and going for the last few days. I am now convinced more than ever that it is an electronics issue. Was leaning towards investigating the intake gaskets, but that is not something that could possibly heal itself. As part of looking for a solution, I will probably have the PCM re-flashed.... GM friend has offered.... I think that fuel pressure is not likely to be an issue either, since that would affect more than one cylinder, and this has consistently been #5, with a bit of #4 thrown in for good measure. Recall that I tested it with engine running, which the manual says could result in readings 7PSI below the "Key-on-engine-off value. Depending on the accuracy of my not-too-expensive gauge, 50 seems within reason, that would put the expected static reading at or above 55. There is no side-arcing from the plug wires, and I have observed in darkness. When a wire is disconnected from a coil with engine running, it can jump a blue-white spark about an inch between the two. Excellent spark, I'd say, at least as far as the "juice" part goes. The timing of the zap.... that I can't feel so confident about. This will have to wait, though, headed out of town for a week. Thanks for all the suggestions so far, will post again when I know more. 
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AH HA!!!!!!! Well well, if it isn't my old friend coming back to bite --yet another probing mechanic.. Oh yes -- are you ready ---are you sure----------------------------- it is a bad or corroded PCM ground!!! Your PCM fires your injectors by appling or switching the GROUND for the individual injector itself -- Make sure all of the grounds for the PCM are good and not corroded or burnt in any way. Clean them up....I'll be willing to bet that your engine runs better in the rain or in wet or high humidity conditions. I must warn you though, Your fuel pressure is low have it tested with a good gauge. Even 5 PSI low will cause problems -- trust me.
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Replying to: alanrs (Jun 27, 2008 10:54 pm) The spring grounds the shield to engine so that you will certainly not see the side spark and it also means that any spark leak to the shield for sure is lost to engine ground and weakens spark at the point where it should be strongest, i.e. inside the combustion chamber. It also is that some of the heat shields are closer to the exhaust manifold parts and side-arching can happen only on certain plugs, but to see any of them the grounding springs must be removed. Arrie |
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Replying to: pear69 (Jul 01, 2008 8:08 pm) So, today I disconnected the battery and pulled the PCM, checking all connections. Look good. Cleaned a couple of the chassis/engine grounds I could find. No change after re-assembly, set a 300 code. Can you point me to the location of the grounds which the PCM uses? The harness which connects to the PCM is wrapped/bundled into a big snake. The non-GM book I have is not much help with regards to the PCM or wiring. Will be re-testing fuel pressure. Thanks. |
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I have a 03 Suburban 5.3 flex fuel that will periodically stall at idle. Sometimes it idles just fine, nice and smooth, other times, it will cut out periodically at about 1-2 second intervals. When it cuts out, the engine almost stalls, but manages to recover. Once in a while it will stall. This only happens at idle, the car runs great under load. I haven't been able to relate the problem to temperature (i.e. open loop vs. closed loop fuel control). At one point, the car was showing 3 codes, bad MAF, lean bank 1 and lean bank 2. I have replaced the MAF sensor, and all 4 O2 sensors, plus new plugs and wires. New PCV valve too. The codes are gone now, replacing these components smoothed out the engine a lot, but I still have this bizarre periodic stumble at idle. It's almost as if there is a period electrical problem that causes the computer to restart or something. Next step is to check for vacuum leaks on the intake side. Any other suggestions as to what it may be ? BTW, about 130K miles...
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