Hi all, looking for ideas here, I've got a 1996 3.9 V8 that is low on power, I've done a compression test, and they are all good, replaced two stuffed leads, new plugs, air flow meter recoed, checked injectors are all working, timing and firing order are spot on. It struggles to get any acceleration, and has a misfire that i cant get rid of. Any suggestions! Apart from the obvious rag and a match!
40wav,
Jul 23, 8:20am
Replace all leads and check/replace air filter, then check exhaust and CAT if fitted!
40wav,
Jul 23, 8:21am
Are you sure the two replaced leads are on the correct plugs!
paul271,
Jul 23, 8:23am
Sorry, should have been clearer, replaced all the leads, and yes, have checked timing 3 times now.
unclejake,
Jul 23, 9:07am
They used to wear out the camshafts, but I would have thought a compression test would show that.
The cat convertor is worth checking. I had a 4.6 with a miss-fire that turned out to be crank endfloat. The crank was moving enough for the crank sensor to get hit by the flywheel. that one took a bit of working out
snoopy221,
Jul 23, 9:09am
Unca jake could be on the money here-I M H O
elect70,
Jul 24, 1:32am
^^ the 3.9 is theroverV8 withconventionaldizy withinternalsssensor .Cam wear wasfault onsome , usually earlier3.5lCheck dizy for hairlinecracks orsigns oftracking ,cleanHT terminals too . & theadvanceretardhosefromair flow body to dizySure you got the firing order rightwhen replacing leads!
elect70,
Jul 24, 1:33am
^^ the 3.9 is theroverV8 withconventionaldizy withinternalsssensor .Cam wear wasfault onsome , usually earlier3.5lCheck dizy for hairlinecracks orsigns oftracking ,cleanHT terminals too . & thevacumnhosefromair flow body to dizycould be split &causing ign to stay retardedSure you got the firing order rightwhen replacing leads!
bellky,
Jul 24, 1:37am
So you've haven't got cat temp too hot on the dash obviously! Two things I can think of (happened to ours): 1) Cats clogged (we replaced with hotdogs no problem). 2) Maf sensor dicky (we got a 2nd hand one no problem).
steve98h,
Jul 25, 8:19am
3.9s are also common for stretchn timing chain but that wouldnt be suddon and the fuel economy goes out the window haha
paul271,
Jul 25, 8:42am
MIGHT be a cam fault, took the rocker covers off, found all the valves are up apart from one, goingto crank it over slowly tomorrow and see what the valves are doing .
kingfisher21,
Jul 25, 10:56am
We had one at work like that, they are really common for knocking lobes off the cam, this one had the whole complete lobe gone from number one intake, obviously been like it for a while, valve was stuck slightly open. This showed up in a compression test, you mention you done a test and all cylinders were good! Still might have a lobe gone, see if all valves open and close while someone cranks it.
mrcat1,
Jul 25, 11:20am
Did they ever have a lot of power to start with!
morrisman1,
Jul 25, 11:28am
the 3.9 was designed purely to burn petrol and nothing else. It was just an unwanted side-effect that it happened to move something
flat_white_ltd,
Jul 25, 11:39am
lol. yup, still wincing from the last one here. $$$ouch !
snoopy221,
Jul 26, 6:50am
wear out the camshafts
paul271,
Jul 27, 7:12am
Found the issues, but customer didnt want to fix them. 1 Vacuum advance not working, 2 sticking valves in l/h bank, 3 oil leaks (numerous!) leaking frost plugs, and vacuum leaks around inlet manifold. Also suspect something was badly worn inside the engine, as there was a suspect clacking noise coming from deep inside the engine. Personally I think he made the right call!
elect70,
Jul 28, 4:02am
^^ wiseI looked at a RRfor$4ghe had spent$6500 on a "recon ' . Sort of thing to own only if can do own repairs . I Bought 3.9Disco with blown head $1500,new gaskets& 1 valve$250&been good since
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