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GENE
GENE, AUTO MECHANIC
Category: Chevy
Satisfied Customers: 801
Experience:  40 YEARS AS A AUTOMOBILE TECH,ASE MASTER CERTIFIED, GM CERTIFIED
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Chevy S10 Blazer 4x4: 4.3L..mpg..s not dumping it to ground..exhaust

Customer Question

I have a ''92 Chevy S10 Blazer 4x4 with the 4.3L Vortec. The vehicle is consuming gas at an incredible rate (~4-6 mpg). It''s not dumping it to ground and I don''t have black exhaust. You can almost literally see the fuel gauge dropping. It starts right up, idles great, and has fair pick-up. No check engine lights. Timing is 0 degrees TDC. Intake vacuum is great. Compression is slightly low (~150 psig) but basically equal across all cylinders (~150k mileage). Opened the upper intake manifold and the passenger side lower intake manifold was cleaner than the drivers side. With fuel pump energized, no leaks observed. Replaced the "spider assembly" anyway. Ouch, pricey! Also, I think the coolant temp. is low. Replaced the thermostat and and temp. sending unit but still seems to be low with vehicle fully warmed up. Any ideas?
Submitted: 6 years ago.
Category: Chevy
Expert:  GENE replied 6 years ago.

wow thats really low fuel mileage-i know this sounds simple-but check the air filter?also do you have access to a scanner with data programs in it?also does the oil have gas in it?the converter will cause this but the o-2 sensors may be adding to the problem-get back to me with this info and we can go from there-also did this all of a sudden start doing this(the fuel mileage) or did it go down a liitle at a time?did you replace the cooling temp sesor or the coolant sending unit?let me know

gene

Customer: replied 6 years ago.
This will probably be more information than you want but...This is my son's Blazer. He asked me to check on an oil leak he had, oil all over front left of engine compartment. He also had a check engine light for his O2 sensor (more on this later) Turns out it wasn't oil but power steering fluid from a bad high pressue hose. Changed out the hoses and p/s pump and while I was at it changed out the oil supply/return lines to the oil filter (fender wall mounted filter). While doing this noticed that the rubber in the motor mounts was virtually non-existent. Replaced the motor mounts. I also went through all of his vacuum lines from his front wheel drive differential vacuum actuator to his transmission(and all points in between). While replacing the vacuum lines I noticed that the O2 sensor lead wires (that run down by the distributor and across the bell housing to the sensor) had been severed. I don't know if they were cut(by someone?, why?) or if with the bad motor mounts the engine had flexed back against the firewall, cutting the wires (is that even possible?). Anyway, I purchased a new sensor lead harness from Chevy and a sensor and wired it back up. No more check engine light. Now, my question is, if I wired it up incorectly (I don't think I did), would I get a check engine light or could it be sending some weird signals to the ECM and still not be showing a check engine light? As far as the catalytic converter is concerned, I ran the motor at idle until at temperature and checked the inlet/ oultet temps and the outlet temperature was about 30 degrees cooler then the inlet (using a handheld IR thermometer). My understanding is that the outlet temperature should be considerably higher if the catalytic conversion "kicks off". I didn't check it at a higher RPM (2500 RPM?).
In answer to your other questions, I don't have gas in my oil, the air filter is new, and I did change out the temp. sending unit and the thermostat (195F). It does seem to be indicating a low coolant temp (170-180F). Actually, I bought a new '93 Silverado with the 4.3L and after a few years my coolant temp. reading dropped. I changed out the temp. sending unit and thermostat with no improvement. It didn't seem to affect the performance so I didn't worry about it. I had a friend with a Chevy S10, 4.3L, same thing???? Have you heard of this before? But, back to the Blazer.
My son wasn't in immediate need of the Blazer so I was able to work through these problems over a few months, as time permitted. Between the time that we parked it and when we started driving it again the tags expired, so I'm stuck test driving it in the neighborhood (30 mph). I've had the battery disconnected multiple times and I believe you have to put a vehicle through it's paces for the computer to relearn the optimal settings, but could that cause such terrible gas mileage?
Hope this helps. Thanks.
Expert:  GENE replied 6 years ago.

well air and fuel has to come into the engine at the correct time-if the correct amount of air is not running throught the motor then theres the problem-having the o-2 sensors disconnected could have ruined the cat-the correct procedure to test a cat is to drill two holes in the exhaust pipe-one in front of the cat and then one behind-using a pressure gauge inserted into the holes(you can remove the o-2 sensor for one of the holes-test to see how much pressure is in front of the converter as apossed to the rear-there should not be much difference in each readings-now having said that-if the o-2 sensors are wired incorrectly then this maybe a problem-im not sure how many wires are on this system-but im assuming there are four-if they were wired incorrectly then it should set a code -but the 92,s were of limited capibilitys so it may not-but it should-what ya need here is a diagnoistic scanner with data programs in it so you can look at what the o-2 sensors are doing and also what other systems are telling the computer-you have to remember that bad info in means bad info out as far as the ecm(computer) is concerned-this may be needed to be taken somewhere where they have a good scanner to see what the systems are telling the ecm-im assumming here that you dont have a scanner but if you do then we can nail this down further-i think that you should test the converter first-a heat test is not always the right way to do this-ive had better success with the pressure test-if you need more just ask-this is a very interesting case and id be more than happy to help

gene

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GENE
GENE
Chevy Mechanic
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40 YEARS AS A AUTOMOBILE TECH,ASE MASTER CERTIFIED, GM CERTIFIED