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MPG woes


BEMSTER 3
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Hello everyone. I just filled my tank up and did my first mpg calculation only to find I am making a little more than 13 mpg in my 1990 coupe, mostly city driving. I know these cars aren't exactly known for their efficiency but our 2015 Silverado 2500 gets better mileage than this. Is there something wrong with my car or is this an average mileage rating?

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My around town driving is usually right around 19-21 and on straight highway driving 27 plus. My best was 32 but I was driving mostly at the speed limit or just below. You could try the usual things, new plugs, wires, O2 sensor and maybe something like Sea Foam thrugh the intake to clean things up.

 I just had the engine rebuilt on the Red and I'm running a 1988 cam with higher compression pistons and my MPG is still in the range I justed stated.

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I just checked the codes (for the first time, I must admit) and the only current code I got was for the cruise control. I've been meaning to replace the spark plugs anyways since the car always seems to start poorly, so I'll try to do that next weekend. Could fuel grade be another factor here? I've been using 87 octane since I got the car a month ago.

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Just got back from my first gas-up after new O2 sensor (and oil pressure sensor) and mileage on my 91 coupe with 135k miles went from about 11 to almost 14 on mostly city driving.  I accept it happily, but not nearly the kind of increase I wanted.  Where do I go from here......knock sensor, new injectors, plugs/wires, coils with or without ICM, MAF sensor cleaning, Seafoam?...I don't know what to do next.  I'm jealous of the rest of you "high mileage" Reattos out there.

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Here is what I would do on a car with the CRT. I'm sure there is a way to do it on your '91 but I can't tell you how.

 

Drive the car until it is completely warmed up. Then go into diagnostics with the engine running. While driving the car check to see if the ECM is going into "Closed Loop" operation and is staying there.  Also check to make sure the torque converter is locking above about 47 MPH. Both of those things can have a big effect your gas mileage.

 

 

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On 4/8/2018 at 8:56 AM, BEMSTER 3 said:

Hello everyone. I just filled my tank up and did my first mpg calculation only to find I am making a little more than 13 mpg in my 1990 coupe, mostly city driving. I know these cars aren't exactly known for their efficiency but our 2015 Silverado 2500 gets better mileage than this. Is there something wrong with my car or is this an average mileage rating?

 

What city are you in?  That's about what I get in stop-n-go Los Angeles traffic (highway at 28+ mpg).

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Brooklyn, New York....I know, I know........at least I don't have to be in LA.  Ronnie, I will try checking ECM closed loop if I can, but how can I " check to make sure the torque converter is locking above about 47 MPH"?  Is it just a sound or feel in the engine while at that speed?  Ronnie you once suggested I might have a leaky injector, any simple way to check for that and are injecters a do-able replacement for a novice-ish guy like myself or will it require my mechanic?  Thanks

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Models with a CRT have an indicator in diagnostics that shows when the converter is locked. I don't know if later models have an indicator for that.

 

While traveling on level ground at about 50 mph, keep your foot on the accelerator maintain speed and slightly depress the brake pedal with your other foot. You should see the tachometer go up a few hundred RPM when the converter unlocks.

 

Injectors aren't hard to replace but you should probably leave it up to a mechanic if you've not had any experience doing it before. You don't want to take a chance with having a fuel leak if you screw something up.

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A bit obscure but need to learn diagnostics anyway. 90 is different from 88-89 but still there. Believe Ronnie has them posted. R code is airbag (90-91). ED04 is coolant temp (in C). Below about 77C/170F (reading) it will drop out of closed loop and the cruise control will stop working. If the lockup fails or the cam magnet falls off you will get an engine error code (21 and 43 AFAIR) .

 

87 octane is best particularly at sea level (85 in Denver). No point in any higher.

 

If all that is good +1 on changing plugs & O2 sensor (Delco)

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Finally started replacing the spark plugs. Only half way there, but if the old plugs are any indication it looks like replacement was long overdue...

 

Now I'm no expert, but I don't think it takes an expert to tell these plugs were in pretty bad shape. Hopefully I'll solve my hard starting and mileage issues once I get the back three replaced as well.

20180414_193713.jpg

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I ended up replacing the plug wires and ignition coils as well as the plugs themselves and... no change. It still starts hard, has a strong smelling exhaust (a symptom I forgot to mention earlier), and based on some very rough math I did from the fuel gauge mileage is the same. I suppose I will tackle the o2 sensor next. I am assuming all three of these symptoms are related, does anything come to mind that could be causing them?

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  Looks more and more like a stuck or leaky injector.

When the ignition is turned off it takes awhile for the pressure in the fuel manifold to bleed down and if an injector is leaking or stuck open a lot of fuel can enter the manifold and give you the symptoms you describe.

  You might try this: First put a pressure gauge on the fitting that sticks up out of the plastic engine cover. About 45 psi with key on and engine not running. Note that when the key is turned on the fuel pump runs for two seconds and then shuts off if the engine does not start - for safety reasons but the pressure in the manifold should hold. Pressure drops to about 36 psi running.

  Lacking a pressure gauge that fits the fuel rail you can try this: Run the car and warm it up. Then shut it off and immediately release the pressure in the fuel rail. Take the cap off the fuel pressure fitting and press the center post in the middle of it.  CAUTION Wrap a rag around the fitting to catch the fuel. Take the necessary precautions or take it to a shop. After doing this let it sit awhile or over night. Does it start and run better or differently?

  Is there a Dr. Injector nearby? That might be the next step. Clean and balance the injectors.

  One troubleshooting tip I forgot to mention is in diagnostics. You have the ability to shut off each injector one at a time and observe the effect on engine idle. Get into diagnostics and with the engine running scroll to the Override section. I believe the keystrokes are explained in the Reatta Quick Reference Guide. This can reveal the problem as well as which injector is suspect.

  One more thing to check could be the fuel pressure regulator vacuum diaphragm on the fuel rail. Could be leaking internally.

Good luck. Let us know what you find.

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  • 2 weeks later...

Just a small update as a bit has happened to the car since I last posted. I decided to take it to a shop to get a professional opinion, and they replaced the o2 sensor, cleaned the injectors, and replaced the fuel filter. Since then the strong exhaust scent has gone away but the mileage does not seem to have improved appreciably and it is starting even worse than ever. This morning I was worried it wouldn't start at all, but it did eventually after about 4 or 5 seconds. I'm not sure how to proceed now and I am afraid to put more money in the car if it isn't likely to solve the issue. I'm also worried that one of these days it isn't going to start at all. The only upside is that it does seem to start ok after it has warmed up.

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3 hours ago, BEMSTER 3 said:

This morning I was worried it wouldn't start at all, but it did eventually after about 4 or 5 seconds.

 

That problem could be a bad fuel pump relay.  If the relay is bad the engine will still start after cranking long enough for the engine to build OIL pressure.  On '90 models the fuel pump relay is located in the interior fuse/relay block. There are test you could do to see if the relay is bad but the easiest thing to do is replace it to eliminate it as being the problem. They are inexpensive.

 

Please add the model of your Reatta to your signature line. It is hard to help you if we have to repeatedly ask what model you have. Although they look alike, each year differences.    HOW TO EDIT YOUR SIGNATURE LINE

 

 

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  • 2 weeks later...

I swapped a few relays around and didn't see any change. I also tried once without any relay plugged into the fuel pump and saw no difference. The car has still not failed to start but has been very intermittent. Yesterday it almost stranded me at a thrift store but today it's starting just as well as it usually does (not well, that is). Should I replace the relay anyways and see if there is a difference?

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Are you sure you don't have a vacuum leak? There are several small hoses that connect the transmission modulator, charcoal canister, fuel pressure regulator, vacuum reservoir for the cruise control and HVAC and the big one easy to miss is the PCV system buried at the passengers side rear of the intake plenum.

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This is a good time to be looking at the built in diagnostics. Bad relay may make it slow to start but has nothing to do with running

 

For example are there any stored error codes ? What is the warmed up coolant temperature, closed loop status, instant and long term fuel mixtures, O2 transitions, & timing advance at about 30 mph cruise ? YWTK.

 

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On 5/1/2018 at 5:02 PM, BEMSTER 3 said:

Since then the strong exhaust scent has gone away but the mileage does not seem to have improved appreciably and it is starting even worse than ever. 

I should probably clarify at this point that I based this assumption on the mileage not improving by how far the gas gauge had moved, not by an actual calculation. I now know that my Reatta is getting almost 22 in-town, having used a full tank. The starting issue seems to be unrelated completely. 

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On 5/13/2018 at 11:45 AM, BEMSTER 3 said:

I also tried once without any relay plugged into the fuel pump and saw no difference.

 

I believe that confirms my theory that the fuel pump isn't being powered by the fuel pump relay but is being powered through the oil pressure sender.

 

Even without the fuel pump relay installed the fuel pump will be powered by a circuit thorough the oil pressure sender once the engine turns enough to build oil pressure. Take a look at the circuit below. The part of the circuit highlighted in blue is the normal route the power takes to the fuel pump. I believe your fuel pump is being powered by the circuit highlighted in yellow. The engine has to crank over enough to build a small amount of oil pressure before that circuit is completed. That would explain your extended crank times for the engine to start.

 

You can test my theory on what is happening by unplugging the oil pressure sender. If the engine will not start you have proved the fuel pressure relay circuit is bad.

 

fuel_pump_circui-2t.jpg

oil_pressure_sensor_location.jpgOil pressure sender location.
 

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