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Corvette
C4 Forum
1988 Convertible for sunny day drives
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<blockquote data-quote="Tourmax" data-source="post: 172280" data-attributes="member: 5304"><p>Car has had a pretty bad knock issue with light throttle since the day we bought it. After a full tank of Shell 93, bad gas was ruled out and it would still knock like crazy with light throttle and higher gears. It knocked so badly it was either get your foot right out of it or you needed to get into it enough to make the 700R4 shift down.</p><p></p><p>Well, that just wont do. This car is intended to be used as a "GT" (ie: grand touring) car, so we needed it to be able to loaf along at light throttle openings.</p><p></p><p>Having images of breaking down that horrendous TPI intake to get to the EGR ports to clean out the passages and the EGR valve pintle, I looked around a bit more before tearing into it.</p><p></p><p>The knock sensor is hooked up and there's no active or stored codes. But the blade connector is pretty cruddy. Knowing how an knock sensor works (generates a small current when it "knocks" which is read by the ECM) and that higher than normal resistance in a circuit can effect operations, I pulled the connector and polished up the terminals. Simple, easy, costs nothing so why not?</p><p></p><p>Then I pulled the MAF and gave it a good cleaning with some MAF cleaner. A PO had put a K&N in the car so it's not unreasonable to think that the MAF would have a coating of oil and general schmutz on it. This is a problem as the MAF works on heating a wire, which is cooled by the airflow. The ECM reads how much current is required to heat the wire to know how many Grams of air are being consumed. The coating on the MAF wire essentially "insulates" the MAF, which means the ECM gets the wrong current reading. Consequently, the ECM schedules the wrong fuel and spark. With the ECM readng wrong, less fuel and spark leads to a lean condition and knock is often the result. Cost 10 bucks for a can of cleaner, so it's also cheap and easy. Just make sure you remove the MAF when cleaning. Maf cleaner and electricity doesn't play well together. Also, you need to let the MAF completely dry before reinstalling it. Count on letting it sit a good 15-30 mins before putting it back in. To cean the MAF, you need to really soak down the sensor. I used nearly an entire can on mine, over multiple cleaning sessions. Took me about 1.5 hrs before I was satisfied it was as clean as I could get it.</p><p></p><p>Lastly, the car had been sitting most of it's life and drives had been short and quick. Very easy to build up carbon in the intake and cylinders. So, as much as I don't like "mechanic in a can" solutions, I dumped a can of seafoam in the tank and then ran a can of Seafoam through the intake via a vacuum port. Well, Ive seen top end cleaners make smoke before, but this was simply EPIC. No doubt in my mind that years of short, go nowhere trips had choked up everything that dealt with combustion gases on this poor little L98. After choking the engine down to stall and letting it sit for 1/2 hr, I took it for a nice long drive. Man, it must have taken a good 10 miles before the engine finally stopped smoking like the rings were fried.</p><p></p><p>But now; no smoke, no rattling and most importantly - no knock!</p><p></p><p>I can still <em>FORCE</em> it into an almost undetectable knock if I lug it very badly, but that's not unusual in those conditions. Either way, I seem to have gotten whatever was causing the knock condition in normal operations. Now it's just a matter of making sure the car goes for a good 20-odd minutes on each drive (with some high speed/highway work) to keep it good and clean inside, as well as continue to clean out what is left in there. MAchines don't like to sit or be lightly used, they want to <em><strong>RUN</strong></em>.....</p><p></p><p>This winter, when the car goes into long term storage, I'll still pull the intake off and give the passages a good, deep cleaning. I'd like to say the TPI will stay off and the LT1 intake will go on, but I just can't see it being done and up to my standards before next spring......<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink ;)" loading="lazy" data-shortname=";)" /></p></blockquote><p></p>
[QUOTE="Tourmax, post: 172280, member: 5304"] Car has had a pretty bad knock issue with light throttle since the day we bought it. After a full tank of Shell 93, bad gas was ruled out and it would still knock like crazy with light throttle and higher gears. It knocked so badly it was either get your foot right out of it or you needed to get into it enough to make the 700R4 shift down. Well, that just wont do. This car is intended to be used as a "GT" (ie: grand touring) car, so we needed it to be able to loaf along at light throttle openings. Having images of breaking down that horrendous TPI intake to get to the EGR ports to clean out the passages and the EGR valve pintle, I looked around a bit more before tearing into it. The knock sensor is hooked up and there's no active or stored codes. But the blade connector is pretty cruddy. Knowing how an knock sensor works (generates a small current when it "knocks" which is read by the ECM) and that higher than normal resistance in a circuit can effect operations, I pulled the connector and polished up the terminals. Simple, easy, costs nothing so why not? Then I pulled the MAF and gave it a good cleaning with some MAF cleaner. A PO had put a K&N in the car so it's not unreasonable to think that the MAF would have a coating of oil and general schmutz on it. This is a problem as the MAF works on heating a wire, which is cooled by the airflow. The ECM reads how much current is required to heat the wire to know how many Grams of air are being consumed. The coating on the MAF wire essentially "insulates" the MAF, which means the ECM gets the wrong current reading. Consequently, the ECM schedules the wrong fuel and spark. With the ECM readng wrong, less fuel and spark leads to a lean condition and knock is often the result. Cost 10 bucks for a can of cleaner, so it's also cheap and easy. Just make sure you remove the MAF when cleaning. Maf cleaner and electricity doesn't play well together. Also, you need to let the MAF completely dry before reinstalling it. Count on letting it sit a good 15-30 mins before putting it back in. To cean the MAF, you need to really soak down the sensor. I used nearly an entire can on mine, over multiple cleaning sessions. Took me about 1.5 hrs before I was satisfied it was as clean as I could get it. Lastly, the car had been sitting most of it's life and drives had been short and quick. Very easy to build up carbon in the intake and cylinders. So, as much as I don't like "mechanic in a can" solutions, I dumped a can of seafoam in the tank and then ran a can of Seafoam through the intake via a vacuum port. Well, Ive seen top end cleaners make smoke before, but this was simply EPIC. No doubt in my mind that years of short, go nowhere trips had choked up everything that dealt with combustion gases on this poor little L98. After choking the engine down to stall and letting it sit for 1/2 hr, I took it for a nice long drive. Man, it must have taken a good 10 miles before the engine finally stopped smoking like the rings were fried. But now; no smoke, no rattling and most importantly - no knock! I can still [I]FORCE[/I] it into an almost undetectable knock if I lug it very badly, but that's not unusual in those conditions. Either way, I seem to have gotten whatever was causing the knock condition in normal operations. Now it's just a matter of making sure the car goes for a good 20-odd minutes on each drive (with some high speed/highway work) to keep it good and clean inside, as well as continue to clean out what is left in there. MAchines don't like to sit or be lightly used, they want to [I][B]RUN[/B][/I]..... This winter, when the car goes into long term storage, I'll still pull the intake off and give the passages a good, deep cleaning. I'd like to say the TPI will stay off and the LT1 intake will go on, but I just can't see it being done and up to my standards before next spring......;) [/QUOTE]
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Corvette
C4 Forum
1988 Convertible for sunny day drives
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