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#387 March 30 6:48 PM
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1999 Dodge 1500 318 automatic. Gas mileage had been deteriorating over several months, and was down around 15 mpg highway and 10 city. Last year, I changed the spark plugs, distributor cap, and rotor; more recently, I had the cat replaced due to a fault code and a temperature test that revealed a much hotter temp entering the cat than exiting it. But mileage didn't improve.

However, I may have inadvertently solved the problem earlier this month. With the hood up while it was running, I felt that the #1 and #3 spark plug wires were pulsing. When I pulled the # 3 wire, that plug was loose, so I removed it and cleaned some fouling. I did the same with the #1 plug, though it wasn't loose, nor was it as badly fouled. After that, the truck returned 17.5 mpg after the first fill-up on the way north. Only one more reading during the round trip was above 17 mpg, but given the windy and cold conditions, with some freezing rain, that should have been expected. It returned above 16 mpg for most of the trip, and about 13 mpg during city driving up north.


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Did you change the spark plug wires?

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1 member likes this: Bob Lincoln
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Exactly. I change them every 30K miles.

If the catcon failed, check the PCV valve and its vacuum diaphragm. The cat clogged in my truck when my brother owned it, and exhaust backed up and melted the transducer.

And make sure the cap has BRASS contacts, not aluminum. Aluminum ones tend to oxidize rapidly in this engine, they form a white powder in the cap towers.

Finally, did you have the original timing chain replaced with the improved chain and guide?


1984 Dodge Daytona Turbo Z 2.2L 5-speed 160K miles

1992 Dodge Dakota 3.9L V-6 4-spd auto 4WD 241K miles; 2011 Chrysler 200 2.4L 6-speed auto 133K miles

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scrounge #395 March 31 12:47 PM
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"Did you change the spark plug wires?"

Except for the #1 wire, no. When I connected the tester to each one, they all flashed strong, so I didn't see the need to, at least yet. The truck regularly returned over 18 mpg with the E3 plugs, but they were fouled; after I replaced them with Champions, 17.5 is about the best it can do. Some of the Champions were fouled pretty bad when I replaced them, though that was after more than 30K miles.

"If the catcon failed, check the PCV valve and its vacuum diaphragm. The cat clogged in my truck when my brother owned it, and exhaust backed up and melted the transducer."

The PCV valve seems okay. The problem might be that the intake plenum gasket needs replacing. The truck uses a lot of oil, and some appears to be is in the tray when I looked through the throttle body. I checked the valve cover gaskets, and it looks like they were replaced with stronger after-market ones at some point; they're not leaking. Also, the original cat looked like it had ripped open, as if it bottomed out over a boulder or something, and was welded shut. A dent remained, which might have impeded flow.

"And make sure the cap has BRASS contacts, not aluminum. Aluminum ones tend to oxidize rapidly in this engine, they form a white powder in the cap towers."

Okay, I'll look. I bought the cap and rotor at a CarQuest.

"Finally, did you have the original timing chain replaced with the improved chain and guide?"

No.


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I suppose this is totally post-vacuum-hose era? (I'm also wondering about universal things like tire pressure, transmission fluid level, sticking caliper...)

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Originally Posted by DanBeans
I suppose this is totally post-vacuum-hose era? (I'm also wondering about universal things like tire pressure, transmission fluid level, sticking caliper...)

I usually check the vacuum lines first; I had to replace a few on the passenger side after buying the truck, but the rest seem to be good. I keep the tires near 40 psi. The trans fluid leaks from the bell housing (probably a seal), which I check periodically and replenish when necessary. I replaced the sticking passenger side caliper last year.

I don't know what effect on mileage replacing the intake plenum gasket will have. From various instructions and videos, that's apparently an all-day job, and setting aside an entire day for it is currently challenging.


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Did you try one of those tricks like spraying something around the gasket to see if it's really leaking?

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Dave #423 April 05 9:18 PM
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Originally Posted by Dave
Did you try one of those tricks like spraying something around the gasket to see if it's really leaking?

Not yet. Saturday, I hosted a garage sale, and today, I had to have a tooth implant, so vehicle diagnoses and repairs have to wait. Fortunately, the truck remains reliable.

Though I cleaned throttle body from above after I bought the truck, it's possible that it and the IAC need a more thorough cleaning:


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I just finished cleaning the throttle body according to the instructions in the video. He said that the throttle body bolts are 13 mm, but I found a 1/2" socket to be a better fit. The long vacuum line connects to the throttle body with a short rubber hose; mine was cracked. I don't know if it was like that, or if I cracked it during removal; it was hard from more than 20 years of use in the Texas weather, so it needed replacing anyway. When I bought a foot of that hose at O'Reilly, I talked with the clerk about how best to clean the unit. He recommended throttle body cleaner specifically, and said that I should soak the caked spots for about a minute before cleaning them. I had Gumout and mass air flow cleaner; he advised against the latter. I bought more carb cleaner at WalMart, as they were out of (or maybe don't stock) throttle body cleaner; I also used some xylene that I had. Both seemed to work well.

I noticed a crack in the air box where it meets the throttle body, so I replaced it with the older one that doesn't fit perfectly over the filter. I'll try to find a better one during the next junkyard run. And the air filter is already somewhat dirty after only a few months use. The one I originally installed stayed clean for a long time. Two houses are being built toward the end of this street, and dirt from that construction might be the culprit.

Whether gas mileage improves will have to wait for my next Michigan trip, scheduled for early next week. The truck at least starts ok.


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Gas mileage didn't improve; the best run was 17.3 mpg, and that was mostly at speed limits of 65 mph or lower. It returned less than 17 mpg during the rest of the trip, less than 16 once.

Acceleration seems to be better, but oil consumption is worse. I read somewhere that the bolts that connect the plenum pan to the intake are too long in Mopars of the era, which is why the gasket leaks, and is why a plenum kit is recommended. If so, wouldn't washers on the bolts work just as well?

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scrounge #464 April 15 12:43 PM
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Yes, that should fix that problem.


1984 Dodge Daytona Turbo Z 2.2L 5-speed 160K miles

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I'm wondering if oil consumption is related to how fast I drive. At fill-ups after speeds of 70 mph or higher (75 in Texas), I had to add a quart, but when top speed was 65 mph, it hardly used any oil. City driving here in Metro Detroit is also using little oil. Is this another symptom of a bad plenum gasket, or might it indicate a different problem?

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Oops, sent the previous post without logging in.


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When I asked about price and availability of the intake manifold gasket set (which includes a plenum gasket), I was told that also replacing the bolts is recommended. Which bolts (plenum or head) weren't specified, but it has something to do with factory torqueing doing something to the original bolts. Does anyone know which bolts, and is replacing them necessary?


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Originally Posted by scrounge
When I asked about price and availability of the intake manifold gasket set (which includes a plenum gasket), I was told that also replacing the bolts is recommended. Which bolts (plenum or head) weren't specified, but it has something to do with factory torqueing doing something to the original bolts. Does anyone know which bolts, and is replacing them necessary?
Usually, the head bolts need to be replaced. When torqued to specifications, they stretch in order to maintain the proper pressure on the head gasket. They do have to be replaced with new ones.

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So just the head bolts, not the plenum ones? And how many and what size? Or are they sold in sets?


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Originally Posted by scrounge
So just the head bolts, not the plenum ones? And how many and what size? Or are they sold in sets?

I've never heard of the plenum bolts needing to be replaced; but I could be wrong here.

I've always seen the head bolts sold in sets for the particular application.

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scrounge #770 September 27 7:06 AM
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It's my first time come here

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scrounge #780 September 29 2:06 AM
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Great forum

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scrounge #849 October 24 10:09 AM
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Glad to see you here!

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