HELP!!! 7.4L problem

MNBowTie · February 3, 2002

If anyone has any Ideas please pass along. Here is the history.

98 7.4L 62k

Cold start no problems until engine management kicks in. towing no problems, no noticable performance loss driving, just a crappy idle and int. missfire.

Devoloped a rough idle and misfire at about 53K.

Step 1. replace plugs (Bosch+4) $79

Result: slight impove, but still have int. miss.

New problem: stalls in carwash.

Step 2. replace 8 plug wires with top quality 8.5MM (Jacobs electronics ) $65

Result: Good, no stall in carwash, However, still have rough idle and int. miss.

Step 3. Go to local precision tune for diagnostics. They find no codes, maybe injector # 2 running a little lean. They recommend fuel induction cleaning I agree $89

RESULT: No impovement.

Bring to local Chevy dealer, they replace two injectors and found antifreeze leak in intake manifold and repair. (warranty)

Result: No impovement, return to dealer for a double check.

Result: Dealer said no codes, agree rough idle and int. miss, but cant't help. I asked dealer to do improved towing perfomance PCM update. I paid $82

Next Step find new dealer:

New dealer, Tech looks at vehicle and says nothing wrong. I persist did the the tech drive it? dealer said no, but he will and I will go with on test drive. The return call from dealer "I agree and was able to duplicate issue".

Dealer: Replaces all 8 injectors fuel pressure regulator,fuel filter,PVC, distributor cap and rotor and reprograms PCM don't know what was reprogrammed although. $350 (warranty for injectors)

RESULT: Very Slight improvement, dealer tech. said to sell it. (dork) who'd buy it with this idle. PLEASE ANY IDEAS?

I'm think about an oil anylsis to check for a head gasket leak. I also had a suggestion from a different forum to do compression check, to rule out a possible worn cam lobe. If this comes true I be speechless. I have used only synthetic since 1000 mi with change intervel around 2500 mi.

RedDog · February 3, 2002

Is your check engine light on? Anybody check the coil?

I have a 97 454. Where is the distributor cap????

RedDog

Shaners · February 3, 2002

I agree that a compression check sounds like the next place to look. The compression check would indicate worn rings or cylinder walls or bad valve seats. Since you don't mention any oil burn problems (unusual for a big block, because most tend to use some oil but that is normal), I'm guessing your rings are probably OK.

MNBowTie · February 4, 2002

Red Dog and Shanners

Distrubutor cap is near the firewall, should be the same as yours Red dog. No check engine light, no codes. Uses about 1/4 quart of oil at 1500. Visually, I can't see any antifreeze in the oil. Dealer is telling me these 7.4's run this way. Red Dog does yours idle smooth? If I open the hood and put my hand on the intake manifold you can feel it shake and mis and you also hear the engine speed pickup a little.

Thaks for your input guys

RedDog · February 4, 2002

I didn't think that GM still called that a distributor cap. Sure doesn't look like one ???

Mine idles smooth,,, it didn't when the MAF sensor was bad.

I would check the coil.

RedDog

CMNTMXR81 · February 5, 2002

I'm leaning more towards the fuel side but am not fully throwing out the ignition side. Any ignition problems would show up as a perfectly timed misfire/stumble. Since it's a 98, they would've been able to scan it on a TechII and see any misfires or low ignition feed voltage if it were on that side.

Another thing I would ask them to do (and it requires a TechII scanner) would be to run a fuel injector leak down pressure test.

Essentially what this does, is you mount a fuel pressure gauge to the fuel rail, connect the TechII to the data port under the dash and then cylinder by cylinder it cycles each injector. As each cylinder's injector fires, you should see the pressure gauge drop a few psi (roughly 5 psi). This tells you that the injector is firing and drawing fuel out of the rail. If no pressure loss is observed, this is the faulty or non-working injector.

As to the oil issue, Oil consumption is a personality quirk of Big Block motors (which your 7.4 is) regardless of whether they're GM, ford or mopar. I have yet to see a Big Block that either doesn't burn it or spill it on the concrete below. It's just the nature of the beast. 1 quart between oil changes is average on these engines. Be thankful though, ford and dodge standards are worse!

MNBowTie · February 5, 2002

CMNTMXR81,

Thanks for the input, however, they replaced all 8 injectors. I wasn't complaining about oil usage, an earlier reply asked how much oil mine used. I think my oil usage is better than normal compared to people I know with the big the big blocks.

Redog, Do you know if the MAF was showing a code before it was replaced?

Right now the lastes from the dealer is they recommend I return to OEM plugs and Wires. I'm not conveniced, however, because it didn't improve anything after the first plug change. The wires made a minor difference in the idle and it didn't kill in the car wash, so leave well enough alone there. I'm going to check the throttle position sensor next.

Thanks to everyone for your input.

RedDog · February 5, 2002

My check engine light came on when the MAF sensor went south . I guessed that was the problem, and put a Granatelli MAF in it. Drove it about 15 minutes and the light went out, plus had a very smooth idle.

I checked my fuel pressure and it is 56. Truck ran real strong when I increased fuel pressure.

RedDog

NeXuS1 · February 5, 2002

i had that same problem on my 95 earlier this month. changed the cap, rotor, plugs, plug wires and ran some STP complete fuel system cleaner. now it runs like it came off the show room floor.

Shaners · February 5, 2002

i had that same problem on my 95 earlier this month. changed the cap, rotor, plugs, plug wires and ran some STP complete fuel system cleaner. now it runs like it came off the show room floor.

I was wondering how it worked out... I think the STP total fuel system cleaner really works. At least it did on my '89 TBI 350...

CMNTMXR81 · February 6, 2002

CMNTMXR81,

Thanks for the input, however, they replaced all 8 injectors. I wasn't complaining about oil usage, an earlier reply asked how much oil mine used. I think my oil usage is better than normal compared to people I know with the big the big blocks.

Replacing them (as you have) usually solves the problem, but the PCM drives them and cycling them through the TechII will tell if the PCM isn't delivering a signal to any injector(s) by having a fresh injector not cycle.

Really the only way to tell is have someone with AutoTap or access to a TechII look up the DTC (and possibly any "pending DTC's") that is triggering the SES light. Otherwise all we're doing here is taking shots in the dark.

I can't see it being MAF related as the MAF will read two parameters constantly regardless of what the engine is doing (I.E. stumbling). It measures incoming air speed by it's frequency in Hz and then quantity in lbs/hr. Both of these parameters would remain pretty steady as the MAF won't pick up any stoppage in air flow speed (Hz) from a stumbling engine on it's backside. I'm not ruling out the MAF, just questioning it.

I'd like to see if your EGR system is turning on at part throttle to mid throttle loads. If it's not, it could be leaning out too much causing knock, which is then picked up by the knock sensors which will subtract timing advance until the knock is gone, which would describe a stumbling.

I'd also like to get a few O2 readings both pre and post cat.

Just thinking out loud on my keyboard.