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Thread: Low MPG 1988 GM with 302 EFI

  1. #1
    Member Tanner02480's Avatar
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    Default Low MPG 1988 GM with 302 EFI

    Hi guys,
    I would like to resolve the low MPG problem on my car,
    I have a 87 and 88 grand marquis, the 87 one is ok, it run fine and have a good MPG

    but my 88 is wrost on fuel efficeiently the difference between the two cars is importent (around 40%)

    I did a tune up on it : spark plugs, wiring, distributor cap : new, I did the timing adjustement to 10BTC (was a little bit to high)

    no jammed brakes, bearings rotors, pad and calipers are new,

    No check engine light, no smoke, no strange smells

    What can I check or replace to improve the fuel efficiently on this car ?

    I wonder if the o2 sensor or the map sensor doesen't work properly, the check engine light should turn on ?

    thanks, and please excuse my English
    1987 Colony Park LS wagon
    1988 Grand marquis LS sedan

  2. #2
    Beater gonna beat sly's Avatar
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    If the 88 has towing gears, this will probably be the case. Around town driving may be 1 better, but highway driving will be lower. If the rear suspension is sagging and the nose is higher than the tail, it will catch more air under the car when moving down the road and cause more resistance. Check the air filter and snorkel to make sure there's nothing sucked up in it blocking the air. Check the vacuum hose to the MAP sensor. If it's not in top shape, replace it. The only other thing I can think of is to check for fuel leaks with the car running and see if there's a small hole in a fuel line or a leaky seal at the tank. Maybe an injector stuck open? If the oil smells of gas, it could be a bad injector. If the alignment is off, it could kill fuel mileage.

    That's all I can think of right now that you haven't already done or pointed out here.

    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
    rides: 88 MGM (SOLD), 93 Vic, 2000 Crown Vic, 2003 Expedition
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  3. #3
    I'm an air-conditioned gypsy gadget73's Avatar
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    You've got most of the obvious stuff. Definitely check for vacuum leaks. These things run richer when there is a leak.

    There is no check engine light. Worth scanning for codes though. If the smog pump bypass does not work and it keeps pumping air into the back of the cylinder head, the ECM will read the mixture as lean and add extra fuel.

    86 Lincoln Town Car (Galactica).
    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

    91 Lincoln Mark VII LSC SE, triple black (Timewarp) - poly front bushings, KYB struts and shocks, Holley SystemMax1 lower intake, SilverFox AOD valve body,

    1984 Lincoln Continental TurboDiesel - rolls coal

    Quote Originally Posted by phayzer5 View Post
    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

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    I post a lot... knucklehead0202's Avatar
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    Fuel filter is an easy cheap thing to replace so that's worth a shot as well.

  5. #5
    Member EaOutlaw1969's Avatar
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    double check your tire air pressure in case you forgot to check them recently
    2000 MGM LS Palm Beach Edition a work in progress

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    Member Tanner02480's Avatar
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    Quote Originally Posted by sly View Post
    If the 88 has towing gears, this will probably be the case. Around town driving may be 1 better, but highway driving will be lower. If the rear suspension is sagging and the nose is higher than the tail, it will catch more air under the car when moving down the road and cause more resistance. Check the air filter and snorkel to make sure there's nothing sucked up in it blocking the air. Check the vacuum hose to the MAP sensor. If it's not in top shape, replace it. The only other thing I can think of is to check for fuel leaks with the car running and see if there's a small hole in a fuel line or a leaky seal at the tank. Maybe an injector stuck open? If the oil smells of gas, it could be a bad injector. If the alignment is off, it could kill fuel mileage.

    That's all I can think of right now that you haven't already done or pointed out here.
    My 87 and 88 have the same rear axle (3.08), the suspension is ok, the rear is a little bit higher than the front , no fuel leak, I just replaced a broken part of the fuel line, air filter is new, alignment is ok and no fuel in oil, I have to check the vaccum hose to the map sensor, thanks for answer





    Quote Originally Posted by gadget73 View Post
    You've got most of the obvious stuff. Definitely check for vacuum leaks. These things run richer when there is a leak.

    There is no check engine light. Worth scanning for codes though. If the smog pump bypass does not work and it keeps pumping air into the back of the cylinder head, the ECM will read the mixture as lean and add extra fuel.
    How I can check if the smog pump is working ? when I purchased the car, the belt of this pump + AC was missing, and I installed a new one, this is the first thing I did on this car. thanks for answer


    Quote Originally Posted by EaOutlaw1969 View Post
    double check your tire air pressure in case you forgot to check them recently
    Tire pressure is ok, thanks for answer
    1987 Colony Park LS wagon
    1988 Grand marquis LS sedan

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    Member Tanner02480's Avatar
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    I read somewhere that the Engine Coolant Sensor failure could result in a two rich mixture because ECU think that the engine is still cold
    1987 Colony Park LS wagon
    1988 Grand marquis LS sedan

  8. #8
    I'm an air-conditioned gypsy gadget73's Avatar
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    If the smog pump doesn't work properly, you'll get codes during the engine running self-test. Usually its 44 and 94, but 41 & 91 or 42 & 92 depending whats going on. The smog pump can produce codes for a lot of reasons, often the passages in the head will clog up but if you get an always lean code, it may not be shutting off when it needs to. Remove the belt and see if the always lean code goes away.

    A failed temp sensor that is out of range will also code, but you may need to measure it directly to confirm

    http://www.grandmarq.net/oldfuelinjection/page10.html

    86 Lincoln Town Car (Galactica).
    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

    91 Lincoln Mark VII LSC SE, triple black (Timewarp) - poly front bushings, KYB struts and shocks, Holley SystemMax1 lower intake, SilverFox AOD valve body,

    1984 Lincoln Continental TurboDiesel - rolls coal

    Quote Originally Posted by phayzer5 View Post
    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

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    I post a lot... knucklehead0202's Avatar
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    My first '90 and my current '90, with 3.08 and 3.55 gears respectively, got within 1-2mpg different on the highway. Around town need not apply because i don't bother to count that. 3.08 car ran stronger despite higher mileage so not all rules come into play. Your experience may vary...

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    Member Tanner02480's Avatar
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    Quote Originally Posted by gadget73 View Post
    If the smog pump doesn't work properly, you'll get codes during the engine running self-test. Usually its 44 and 94, but 41 & 91 or 42 & 92 depending whats going on. The smog pump can produce codes for a lot of reasons, often the passages in the head will clog up but if you get an always lean code, it may not be shutting off when it needs to. Remove the belt and see if the always lean code goes away.

    A failed temp sensor that is out of range will also code, but you may need to measure it directly to confirm

    http://www.grandmarq.net/oldfuelinjection/page10.html
    Okay, Thanks for answer, is it possible to have these codes and don't have the check engine light on ?

    I will proceed with a diagnostic anyway, I located the test connector
    1987 Colony Park LS wagon
    1988 Grand marquis LS sedan

  11. #11
    I'm an air-conditioned gypsy gadget73's Avatar
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    Some late 88's got a check engine light, but not all. Yours may not have a functional one. Definitely possible to have codes without a light. Very few things actually turn that on.

    86 Lincoln Town Car (Galactica).
    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

    91 Lincoln Mark VII LSC SE, triple black (Timewarp) - poly front bushings, KYB struts and shocks, Holley SystemMax1 lower intake, SilverFox AOD valve body,

    1984 Lincoln Continental TurboDiesel - rolls coal

    Quote Originally Posted by phayzer5 View Post
    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

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    Member Tanner02480's Avatar
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    Quote Originally Posted by gadget73 View Post
    Some late 88's got a check engine light, but not all. Yours may not have a functional one. Definitely possible to have codes without a light. Very few things actually turn that on.
    You right, the check engine light do nothing, it only light during the time I turn the key to START, It's not give me the blinks to tell me the codes, I have no choice to read the codes with a voltmeter, my diagnostic connector is in bad condition ,There is some oxydation on the terminals, I difficultly get codes 21 and 24 but maybe due to the cold temperature this morning, I will take the time to do that in my garage after an engine warm up and diagnostic connector cleanning...
    I will come back with updates next week, Thanks To all
    1987 Colony Park LS wagon
    1988 Grand marquis LS sedan

  13. #13
    I'm an air-conditioned gypsy gadget73's Avatar
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    The engine has to be warmed up in order to test it properly, otherwise you'll get at least a code 21. Cold enough and you may get a 24 as well. If the engine is warm and you get one or both, that could definitely explain some of your issues.

    86 Lincoln Town Car (Galactica).
    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

    91 Lincoln Mark VII LSC SE, triple black (Timewarp) - poly front bushings, KYB struts and shocks, Holley SystemMax1 lower intake, SilverFox AOD valve body,

    1984 Lincoln Continental TurboDiesel - rolls coal

    Quote Originally Posted by phayzer5 View Post
    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

  14. #14
    Member Tanner02480's Avatar
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    Hi guys,
    I just did the tests today after warm up
    Key on code : 11
    Mem code : 33

    Engine running code : 44 94

    The 33 say, it remember one time in the past the egr valve doesn't work

    44 and 94 is the air injection systeme fail (the belt is removed)

    I don't think the air injection system could affect the mpg

    I found and repaired a broken vaccum hose on the air injection (Valve on the upper of the right side exhaust manifold)


    I wonder if the oxygen sensor doesn't work properly, will I had a code for it ?

    I think the trouble could be the o2 sensors or a blocked catalytic converter.

    For remember, my troubles is Low MPG and Low power

    Thanks
    1987 Colony Park LS wagon
    1988 Grand marquis LS sedan

  15. #15
    I'm an air-conditioned gypsy gadget73's Avatar
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    If the belt is off the air pump, it can't affect anything. The only way it can is if the pump is working and constantly pumping air into the back of the head. The ECM will show the fuel mix as lean and add fuel to compensate. No belt just gives that fault code though, no worries.

    Does the code go away with the belt on? If so that proves the O2 sensors work, though it won't work if the passages are blocked up with carbon.


    Could be clogged converters. O2 sensors rarely give fault codes. Not impossible they are bad too. I've had poor fuel economy from a bum O2 sensor with no other faults.

    86 Lincoln Town Car (Galactica).
    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

    91 Lincoln Mark VII LSC SE, triple black (Timewarp) - poly front bushings, KYB struts and shocks, Holley SystemMax1 lower intake, SilverFox AOD valve body,

    1984 Lincoln Continental TurboDiesel - rolls coal

    Quote Originally Posted by phayzer5 View Post
    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

  16. #16
    Member Tanner02480's Avatar
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    With the belt on it only give the code 44 but I noted that only one of the two catalytics is plugged to the smog pump
    1987 Colony Park LS wagon
    1988 Grand marquis LS sedan

  17. #17
    Wagon Addicted Tiggie's Avatar
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    Might want to check EGR function. Good post here: https://www.stangnet.com/mustang-for.../#post-8656265

    Do the two cars run about the same? I noticed a difference from my 88 and 90 in terms of power and MPG; I suspect 88 has some weak compression issue.
    1988 Crown Vic Wagon - daily
    1990 Country Squire - weekend cruiser, former lawn ornament
    Other: 95 Ranger, 74 F250, 68 Mustang, 94 Mustang
    Sold: 1982 LTD and 1987 Crown Vic

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