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I have a 97 Dodge Ram 1500 5.2. I have been getting a check engine light indicating a cyl # 7 misfire detected. The truck runs good before the light goes on, what it does is misses, shortly before the light goes on and while the light is on. This miss is more noticeable at a stop or on acceleration. It usually takes 8 to 10 mi of hwy driving to make it come on otherwise 10 to 20 miles if city driving. This is what we have done to the vehicle, 1st plugs, wires ,cap, rotor, replaced air filter, and check pcv valve and checked and replaces vacuum lines that looked bad. 2nd cleaned fuel rail. Still no
change except it took more miles for the light to come on. 3rd we switched # 3 and #7 injectors around also checked resistance when they were in their respective spots,
checked out good. Again same code. 4th we did a compression test that showed # 7
cyl @ 120lbs dry, 125lbs wet, all other cyl. range dry 135 to 150 & wet 140 to 160. Next
we checked the map sensor w/ a volt ohm meter, the volts were down slightly,
changed it, put new one in then checked w/ meter again, checked out good. left it in.
Drove truck about 12 to 15 mi no light. Drove it next day about 13 mi to work, just before
I got their I noticed at miss at the stop lights, no code by the time I got to work. Worked
10 hrs went home still noticing a miss at stop and start and a slight decrease in power, no light yet. Stopped to get 8 gal of gal and a car wash, while in car wash light
came back on. last I drove it. I'm giving up any suggestions????? Please help!!!
Terri

Optional Information:
1987 Dodge ram 1500 5.2

Already Tried:
tune up, map sensor, compression test, move injectors

Customer (name blocked for privacy)

Status: Closed Value: $8

I have run into some of these engines with misfire codes. The first thing
to look at is a vacuum leak at the
intake lower cover. That can be checked by inducing propane into the crank
case with the pcv valve blocked and look for a change in idle speed. Second
thing to watch for is the valve seats getting wiped out. A lot of the time
the compression will be good as the valve will seat at times and not at
others. Try a running compression test several times on #7 cyl and try to
catch it in the act.

Reply to JONATHAN's Post: Hello, I just want to clarify something with you before we do this. When we induce the propane in to the crank case do you want us to put it in the crank case breather port, put it into the pcv port (with the pcv pulled and taped off) or do you want us to run it along the intake gasket by # 5 and #7 cyl? Just wanted to double check before we do it. If this dont work we will try the compression tests. thanks for your help

run it along the intake gasket around #5 and #7 cylinder....but try this first.......
Plug the driver side valve cover grommet.
Remove the pcv from pass side valve cover and plug pcv
valve. With engine idleing plug pass side grommet with your
finger.If you feel vaccum you have a internal vaccum leak.
This leak is coming from the lower intake pan under the
intake. Dodge has uppgraded the gasket to metal instead of
paper. This leak can cause a lean missfire on one or more
cylinders.

Reply to JONATHAN's Post: Hello, we wanted to get the code back in to the truck, to try these test and more. Sorry it took so long we have been busy. Ok we tried the hand test felt no vaccume leak (pressure only no suction), we tried the propane test, the motor didnt run different. Mind you the check engine lite is on, more noticeabe misfire when vehicle is in drive, but at idle(stop sign). We tried this gaget that you hook between the spark plug with a clip and the wire boot. It measures with a arc between two points and shows a visual of the cylinder firing. We saw a slight hint of a mifire two times, like the arc went away if we rev the rpms up to 1,000 while in drive. The arc was smooth while it was in park. What do you know about crankshaft position sensors, air temp senors and coolent temp to pcm, or do you still think we are looking at the intake yet.If you do thats fine thats what we are trying to figure out. We still need to do the compression test again (like you suggested) waiting for our friend to bring the tester back. What do you think this far.

i doubt its any of the sensors you've mentioned...all of those would effect more than just cylinder #7....i would definitely check running compression...the other thing i didn't mention that could be a possibility is a weak lifter...which a co-worker mentioned to me concerning his truck....keep me posted....

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